Comparative Analysis of Residential Parking Questions in National Travel Surveys: A Cross-Country Study

Abstract

Availability and utilization of residential parking locations remain a crucial yet often overlooked aspect of urban planning and policy making. Thus, this study investigates questions about residential parking within National Travel Surveys (NTS) across 49 OECD-affiliated countries. The study used a layered framework, differentiating parking locations by on/off-street, public/private access, proximity to dwellings, and infrastructure type to analyze questions and response options. A document review revealed significant variability in the inclusion of residential parking questions, with only 24 of 49 evaluated countries conducting NTS and only 15 explicitly addressing residential parking locations, often via one question. While questions are often similar, the response options are often ambiguous, sometimes misleading, and vary significantly. Based on the results, we identified the following recommendations: (1) adopt and rephrase parking-related questions in new and existing NTS, (2) provide guidance to users on how certain response options are defined, (3) for a more comprehensive understanding of residential parking dynamics, include a question on availability and actual usage, (4) add “National Travel Survey” as a keyword in applicable surveys, and (5) provide easy access to documentation to facilitate data retrieval and international comparison.

1. Introduction

Unfortunately, private parking is an often-neglected aspect of (urban) infrastructure data. In 1996, the Association of Towns and Municipalities of North Rhine-Westphalia, Germany’s most populated federal state, wrote in its recommendations on “objectives and components of municipal parking concepts” that the mapping of private parking space is difficult and time-consuming, as the corresponding registers are generally not available. Yet the report highlights its importance for problem assessment and concept development [1]. In 2024, 28 years later, a report concerning urban structure, choice of residential location, and daily mobility formulates “Improving the data basis on private and public parking space supply” as one recommendation for action [2]. These recommendations spanning nearly three decades underscore the need for comprehensive data on private parking spaces for urban planning and policy making. These statements align with Shoup’s argument that “no city collects data on its total parking supply” [3]. In addition to this long-standing gap in knowledge about parking supply in terms of parking requirements and parking management, the demand of the transition to electric vehicles (EV) increases the relevance of knowledge of private parking availability.

On average, a majority of cars are parked at home at any given time, and the duration of parking there is particularly long, making it an ideal location for slow charging an EV [4]. Home charging is typically the most cost-effective and convenient method for EV owners, allowing them to charge their vehicles overnight. Several studies show that most current EV users charge at their homes or workplaces [5,6,7,8]. Most users are likely to reduce the number of charging stops if charging is not as convenient as at home. If the electric range is sufficient, users try to avoid using public chargers [9].

It should be considered that EVs as of today have a disproportionately high share of private parking availability [10], a fact that is obvious with 92% of BEVs are parked on private properties compared to 75% for all private cars [4]. Depending on the number and share of EVs, different shares and distributions of private parking need to be addressed as people who can install a private charging infrastructure are more likely to own or purchase an EV [11,12].

National surveys can serve as a possible source of data to analyze parking for multiple areas and regions, with the limitation that it is not possible to accurately locate all parking facilities, but to make statements for certain groups or spatial units. These can then be used, for example, as a basis for an estimate of the availability of private parking spaces in certain areas. One possible type of such a survey is the National Travel Survey (NTS).

The term NTS, as used here, covers surveys of daily travel on a national basis. These surveys contain information on the socio-demographics of individuals and households and the conditions at the place of residence. Detailed information on vehicles and their use may also be included. Therefore, NTSs appear to be promising potential data sources for obtaining nationwide, but also spatial differentiated information about residential parking. In addition, there are numerous variables that might be used to explain differences in private parking availability. However, differing focuses and questions may result in varying levels of specificity and informative value. This study aims to investigate how the topic of parking is addressed in such surveys and how questions on residential parking locations differ across various countries.

The availability of parking as an element of NTSs was mentioned by [13], who compared NTSs of EU27, the United Kingdom, and Switzerland. Out of 15 surveys included in their analysis, around 50% included information on parking availability at home. However, the exact questions and any distinctions were not analyzed in detail. They also suggested that parking data may be of greater interest in the future due to the introduction of electric vehicles.

To this end, NTS from up to 49 countries are screened with regard to the topic of availability/utilization of parking spots. In view of the variation of parking locations (shown in Methodology), the question arises as to what extent this is being considered in NTSs and other surveys. Therefore, it is necessary to take a closer look at the specific questions and answer options. This results in two main research questions: (1) To what extent is the topic of “parking” included in NTSs? (2) How are the questions specifically asked and what conclusions can be drawn about the availability of parking facilities at the place of residence? The aim of answering these questions is firstly to find out to what extent individual NTSs, initially based on the questions regarding parking, are suitable for use in the analysis of parking facilities. The second aim is to see whether an international comparison based on these data would be possible, as Buehler [14], for example, has carried out in relation to mode choice.

This paper is structured as follows. This section is followed by a literature review on private street parking and an overview of potential other data sources. The methodology section describes the web search procedure, the scope, and a categorization of different resident parking options. The results section describes the survey results and the specific questions and response options. The paper then discusses the implications of the results for significance and potential difficulties in categorizing different parking options. Finally, the conclusion summarizes the key findings and provides recommendations for the implementation of parking in NTSs.

2. Literature and Potential Data Sources

The literature on parking in residential areas rarely deals with the analysis of the quantities of private parking spaces on a large scale. However, residential parking has been studied in different regions and spatial structures in various countries.

2.1. Private Off-Street Parking

With regard to private off-street parking, the focus is on the following topics: (1) parking (minimum) requirements for building construction, (2) effects of parking space availability on car ownership, (3) relationship between on- and off-street parking in residential areas.

In the literature, concerning residential off-street parking, much attention is paid to the impacts of parking requirements. The research covers cities on different continents such as North America [15,16,17,18], Europe [19,20], Asia [21], and Australia [22]. Despite the differences in mobility and settlement structures, the researchers draw similar conclusions. Minimum requirements are often criticized as a driver of vehicle ownership, higher construction costs, and land usage. Manville et al. [15] states that a 10% increase in minimum parking requirements is associated with a 5% increase in vehicles per square mile, a 4% increase in vehicles per person, and a 6% reduction in both population density and housing density. The international relevance and differences of parking policies is also illustrated by Kong et al. [23], who compared the parking policies of 16 countries from different continents.

The topic of parking requirements is strongly related to the topic of the relationship of residential parking to car ownership and travel behavior. In the past, the perspective was that the required parking space was directly related to car ownership, with urban planners believing that as more people owned cars, more parking spaces were needed to accommodate them [24]. Today, however, the understanding has shifted: more parking is seen as inducing higher car ownership, meaning that the availability of parking spaces encourages people to own more cars. Shoup [3] argues that the availability of parking spaces significantly impacts whether households choose to own a car. Guo (2013) supports this view by demonstrating that the presence of residential street parking correlates with higher car ownership rates. Christiansen et al. [25] explores how household parking facilities affect travel behavior and car ownership.

Even if the effects of both on-street and off-street parking have been established, the focus is often on one of these options. The authors of [26] even stated that there is no clear evidence of to what extent off-street parking, or requirements for it, actually offset on-street parking use and analyzed the effects of residential off-street availability on on-street parking in parts of Melbourne, Australia. Underutilization of private parking space can be observed in low-density housing areas, as well as in dense areas. Scheiner et al. [27] emphasized the importance of utilizing under-used private parking spaces in a neighborhood in Dortmund, Germany where just 24% of car-owning households had a private parking facility to alleviate the pressure on public parking infrastructure. Contrary to this, Rogers et al. [28] came to the conclusion that the underutilization of private parking spaces in Washington D.C. (USA) is primarily due to the oversupply of private parking spaces. For detached houses, it is mostly garages that are used for other purposes, such as storage [26,29]. Simićević and Milosavljević [30] explored the impact of parking regulations and discovered that stringent on-street parking regulations encourage drivers to choose off-street parking options. This aligns with the findings of Golias et al. [31], who reported that drivers’ sensitivity to off-street parking options is influenced by regulatory measures and the convenience of these facilities.

What many of these studies have in common is that they usually only relate to parts of certain cities, individual neighborhoods, or buildings constructed at certain times. Few studies have attempted to systematically estimate off-street parking inventories or availability of parking for households or vehicles for an entire city or region. As one of the few examples, [32] developed an inventory of residential and non-residential off-street parking by combining regional land parcel data with municipal off-street parking requirements. Another publication created comprehensive parking inventories (on-street, off-street structured, off-street surface) for five US cities using available street geometries, parcel data, and satellite imagery [33].

2.2. Other Potential Data Sources on Availability of Private Parking Facilities

In the search for suitable sources of information on parking, other potential data sources that could contain information on parking were also considered in addition to NTSs. Some of these enable other evaluations. Others do not take parking facilities into account or are not accessible. Such data sources can be divided into nationwide and local data sources. The former include population and housing censuses, dedicated building and housing surveys, and surveys on living conditions, tax data, and building permits. At a more local level, there are parking inventories, mobility surveys, residential parking requirements, and information based on geodata or imagery.

2.2.1. Nationwide Data

A potential data source are population and housing censuses. These are carried out at regular intervals and are made available in various ways. For the European Union, regulation on population and housing censuses established a minimum standard on topics to be covered, such as type of building or period of construction [34]. Unfortunately, parking lots or garages are not listed as a survey item and have, at least for Germany, not been added when transposed into national law [35]. The same applies to the Irish census, although it at least includes the number of vehicles in the household [36]. Nevertheless, georeferenced results from censuses can play an important role in providing spatially differentiated estimations of the availability of private parking facilities by linking them with results from other surveys.

Surveys on buildings and housing consumption or living conditions might also cover the topic of residential parking. Those surveys can be found all over the world, for example in Chile, France, India, Japan, or the US. [37,38,39,40,41]. The building focus of these surveys presents a challenge when car ownership is not queried, as it yields no insight into the availability of parking specifically for households with vehicles. Scheiner et al. [27] found for a densely populated neighborhood in a German city that only 18.6% of all households but 24.7% of car-owning households have a private parking facility. Consequently, derived proportions may prove unreliable because of significant disparities between all households and those owning vehicles. Another potential downside is the focus on buildings or dwellings, hence in most cases only on-property off-street parking might be considered. In addition, it might be limited to building structures, such as garages, and exclude parking in the driveway. Questions are commonly dichotomous. For example, it is asked whether a house has a garage [42,43,44].

The European Union statistics on income and living conditions (EU-SILC) collect data on housing conditions, harmonized by legislation. Garages or other parking spaces are not included and even the availability of a car is not a core variable. The variable’s existence depends on the implementation in the given member states [45]. Nevertheless, there are European countries, such as Spain, the Netherlands, and England that have conducted dedicated housing surveys in the past, covering residential parking in one way or another [46,47,48].

Additionally, there are instances where data have been collected but are not accessible for researchers or municipalities. A notable example is the declaration of property tax value as part of the property tax reform in Germany. In this case, every homeowner had to declare the number of garages and underground parking spaces [49].

In Canada, information on the number of garage and carport permits issued for residential construction is available for different geographic units, including Census Metropolitan Areas, dating back to the year 2000 [50]. However, this data set has limitations; it does not include information on older existing buildings, the number of uncovered parking spots, or the utilization of those garages as a parking spot. Furthermore, in many countries, this type of structure does not require a permit. For instance, in most states of Germany, uncovered parking spaces, garages, and carports are not subject to a permit [51]. In addition, this type of information might be only available for the individual municipality level in other countries.

2.2.2. Local Data Sources

There are further data sources at a local level. These include parking inventories or survey-based mobility reports. However, these are not available across the board and vary in terms of methodology and the parking facilities analyzed. Often only on-street and specific off-street parking options are recorded [52,53,54]. In some mobility reports [55,56], there are spatially differentiated results on the main residential parking location. There can be a great similarity to NTS here, so that this information could be used, for example, to see to what extent a municipality differs from municipalities of a similar type and size.

As discussed in Section 2.1, parking requirements are used as a planning tool on several continents. It may therefore be possible to estimate the level of private parking via the required parking spots. However, there are several obstacles to this approach. Parking requirements give local authorities the ability to influence the supply of off-street parking when a building is constructed or converted. However, current requirements cannot be applied to existing buildings. Second, regulations differ between cities and can even vary within a single city, depending on factors such as population density, centrality, and public transport accessibility. A recent study has shown that if all existing buildings were built according to current regulations, the parking supply would significantly exceed the demand derived from the vehicle stock in cities with high minimum requirements [57]. This is not consistent with the figures from the English and German NTS [4,58], for example. Data from the English Housing Survey show that the older a building is, the less likely it is that residents will have a parking space [47].

In more recent years, deep learning methods have also made it possible to identify car parks or available parking spaces using image data. Parking inventories generated in this way can contain both on-street and off-street parking options [59]. Parking lot classification in the US is also one of the pretrained deep learning models provided by ESRI for use in ArcGIS [60]. However, underground car parks, for example, cannot be recorded in this way. It is also not always possible to determine who has access to the given of the off-street infrastructure. The calculation is very GPU-intensive, which makes it difficult to use on a large-scale.

Overall, NTS appeared to be the most promising and widespread data source for nationwide analyses. In contrast to housing surveys, it can be expected that parking facilities are also considered in terms of the presence of a car. In addition to the variables describing people, vehicles and households, depending on the design of the survey, neighborhood characteristics may also be considered in relation to the availability of specific parking options.

3. Materials and Methods

In order to analyze the relevance and occurrence of parking in NTS, an in-depth review of questions and corresponding answer options was conducted. Therefore, the scope needed to be defined in regard to spatial and survey context. This section also outlines the procedure for searching and evaluating the surveys by introducing layers of parking locations.

3.1. Scope

The scope of this study includes OECD (Organization for Economic Co-operation and Development) member countries, accession candidates, and key partners, as of July 2024 (

Table 1. Included OECD member countries, accession candidates (C), and key partners (P). Sorted by continents and population.

Africa (1) South Africa Asia (8) India (P) China (P) Indonesia (C) Japan Türkiye Thailand (C) Korea Israel

Europe (29) Germany France United Kingdom Italy Spain Poland Romania (C) Netherlands Belgium Czechia Sweden Portugal Greece Hungary Austria

Switzerland Bulgaria (C) Denmark Finland Norway Slovakia Ireland Croatia (C) Lithuania Slovenia Latvia Estonia Iceland Luxembourg Slovakia

North America (4) US Mexico Canada Costa Rica South America (5) Brazil (C) Colombia Argentina (C) Peru (C) Chile Oceania (2) New Zealand Australia

). These 49 countries represent a wide range of population sizes, motorization rates, GDPs, and other development indicators. Due to the structure of the OECD and the general number of countries by continent, we get an imbalance in regard to countries per continent. With 29 countries, Europe accounts for more than half of the countries surveyed. Those countries represent large parts of Europe, while just four countries cover a large part of North America. Oceania is well represented by Australia and New Zealand in terms of population. In South America, over 80% of the population lives in the five countries included. In Asia, although there are relatively few countries included, the represented population share (>70%) is high. The least represented continent is Africa, where only South Africa is an OECD member.

The focus of this work relied on questions related to parking. The exact questions and response options were of particular relevance. Differences between surveys in regards to design characteristics as have been looked at by various researchers [61,62] are not within the scope of this research.

3.2. Data Collection

In order to collect the surveys of each OECD country, internet research was conducted. English search terms were initially used to research the relevant surveys. If this was unsuccessful, the terms were translated into the respective national language. Websites and reports were then searched through using Google Translate and DeepL. In some cases, the option to translate the entire website was used. If relevant information was found, the translation was always checked individually. To reduce the likelihood of mistranslations and omissions, these tools were used as a two-way dictionary for individual words and to provide translations for selected phrases. This allowed several possible translations to be compared. For countries with a different script, such as Japan and Korea, this approach proved particularly challenging. In these cases, we expect the highest level of uncertainty about a precise translation.

Search terms used for the NTS included terms such as “National (Household) Travel”, “Mobility”, “Transport”, “Traffic”, and “Trip survey”. Even if some of these terms might lead to other types of surveys. In English-speaking countries, the relevant surveys could be found directly using the term “National Travel Survey”. The same applies to other countries, such as Germany, even if the survey itself is named differently. In addition, surveys covering tourism activities were often found under the search term National Travel Survey. This was particularly the case in countries where no NTS could be found, such as Canada [63] and Thailand [64]. In other cases, the search terms led to travel surveys with local or regional coverage.

In some instances, information on NTS carried out could be taken from the literature. A report published by Eurostat [65] gave a good initial overview over some surveys conducted in Europe. Unfortunately, in this case, no references were given for the surveys and some of the information was outdated or inapplicable.

Various elements can be found in the survey’s dissemination. These range from press releases, result reports, and methodological and technical reports, to dataset descriptions, aggregated data tables, and microdata. In multiple cases, specific elements are only available to selected groups. Ideally, the questionnaire, codebook, or technical/methodological report was accessible. The questionnaire in particular is relevant in order to recognize subtle differences between the questions. In the codebooks, the variables are often briefly described, but do not always allow conclusions to be drawn about the question asked. On the other hand, derived variables, especially spatial variables, which are not recorded via the questionnaire, can also be considered in these. In some cases, these elements could only be found on the websites in the national language; in the English version of the website, only the results report could be found. In general, the questionnaire was often particularly difficult to find, especially if it was not included in the appendix of the technical, methodological report.

3.3. Parking Locations

Parking locations can be differentiated in terms of the trip purpose. People tend to park for longer when travelling to work than when shopping. However, vehicles are parked by far the longest at home [4,58,66]. In some cases, parking is only differentiated on this level [6]. In other cases, only on-street and specific types of off-street parking [67], relevant for specific trip purposes such as shopping, are differentiated [68]. As this paper primarily takes residential parking into consideration, we ignore potential parking options at other locations.

Despite looking only at one parking location, there are sometimes very granular distinctions between the various options. Parking can take place in private or public areas. Next to parking on the public street, there are multiple off-street options such as garages, underground garages, or the driveway, parking lot, or parking. Those off-street parking options can be further differentiated. If the vehicle is parked in a private car park or a public underground car park, this is just as much off-street parking as parking in a private underground car park belonging to the apartment building.

Depending on the level of detail required for different analyses, these differences are of different relevance. For research in relation to parking requirements, it does not matter whether parking options are divided into individual garages, parking lots, or driveways. Only the amount of off-street and sometimes on-street parking is of essence [69,70]. In other cases, for example when considering fire protection when installing charging infrastructure, only larger parking facilities in the form of buildings (underground car parking, parking garages) might be relevant [71]. With regard to charging infrastructure equipment, the distance and association to buildings can also be relevant [72]. Others, who looked at the effect of private charging infrastructure on the readiness to buy electric cars, divided residential parking options into private, shared (a multi-car parking lot or garage), and street [12]. Another classification distinguishes private parking options according to their location inside or outside a structure [33]. Overall, it can be seen that a number of different distinctions of parking are used in the literature. We therefore assume that there will also be differences in classification in NTSs.

In order to be able to show as many differences as possible with regard to the parking location, we have opted for a layered classification. Comparable classifications, which however did not always include all options, were also found in the literature. A two-layer classification can be found in Scheiner et al. [27], who divide the private parking options into garage, a parking space in an underground garage, a car port, or another parking space. In addition, a survey was conducted on these parking options. Morency and Tréanpier made a three-layer distinction in terms of jurisdiction (private/public), location (indoor/on the street/outdoor), and rates (free/fee charging/subsidized by the employer) [73].

Our differentiation of the parking location is shown in Figure 1. The division into layers 0-IV is intended to help answer the second research question regarding the survey questions, answer options, and possible conclusions. For this purpose, it is considered at which layers parking spaces were surveyed and whether all available options at a particular layer were considered. Layer 0 is based on trip purpose and destination, as we are just looking at residential parking other locations are not analyzed. Layer I differentiates whether the parking is on or off the street. Layer II is based on access restrictions and differentiates between private, public, or publicly accessible (semi-public). Layer III examines the location in relation to the dwelling, determining whether it is part of the house or if there is some distance between the house and the parking location. Layer IV addresses the type of parking infrastructure, noting that one type of infrastructure can vary regarding the previous levels and might be assessed differently depending on the analysis purpose. Other aspects to differentiate parking including ownership, inclusion in property rent, costs, and the distance from the parking spot to the dwelling are not considered in this paper.

3.4. Reference Levels

In terms of analytical aspects, in addition to the layers of parking locations presented in Figure 1, questions were categorized by their reference level A–C (Figure 2). The reference levels are households (A), persons (B), or vehicles (C). The reference level “person” is often a proxy for either household or vehicle depending on each survey methodology and question phrasing. If each person in a household is asked to indicate the parking location for each vehicle of a household after use of the respective vehicle, other additional conclusions can be drawn.

Different questions can be answered at each of these 3 levels, as illustrated by the examples provided for each level (Figure 2). Questions can either focus on availability or on the actual use of parking spots. If both availability and utilization are queried, conclusions regarding occupancy rates could also be drawn.

4. Results

The results are presented in two steps. In the first step, the countries that have conducted an NTS in the past are identified, with a particular focus on those that have addressed the topic of residential parking. An overview of analyzed NTSs can be found in Table 2. In the second step, the different questions and answer options are described (Table 3). The statements in the results and methodology section refer to the respective surveys that are referenced in Table 2. Should further references be relevant, they will be cited accordingly.

4.1. OECD Affiliated Countries with a NTS

An NTS, according to our definition, was identified in 24 out of 48 countries. In Europe, 18 countries have conducted an NTS, while in Asia, three countries did. North America, Africa, and Oceania had one each, and South America did not have any countries with an NTS. However, the surveys are not evenly distributed across Europe. In particular, countries in eastern and south-eastern Europe do not conduct NTSs (Figure 3). Overall, 13 European NTSs addressed the residential parking location. In addition to the European countries, only New Zealand and Israel NTSs include questions about parking location, resulting in 15 out of 24 NTS covering this topic.

The majority of NTSs have been conducted in the last five years. In some instances, newer iterations, such as the German survey Mobility in Germany 2023 [74] or the English NTS in its 2023 iteration already have been conducted, but the results have not yet been processed. In England, questions alternate between years, so parking is only surveyed in even years. The topic would therefore only be considered in the 2024 NTS.

For some countries, certain aspects must be taken into account. For example, since 2013 the NTS in the UK is limited to English residents [75]. The Israeli national data set is a combination of regional travel surveys conducted between 2010 and 2019. Due to differences in the questions and answer options in each regional survey, variables have been recoded or omitted. The Canadian NTS covers tourist trips. The US does not have a question on parking type as defined in this paper. However, the survey does include other parking-related questions about paying for parking or walking distance from the parking lot to the destination. No documentation was found for Luxembourg, leaving open whether parking was queried.

There may be more than one nationwide travel survey—for example, in the Netherlands and Germany. In Germany, in addition to the Mobility in Germany survey, there is also the German Mobility Panel (discontinued/last iteration in 2022/2023) [76]. In the Netherlands, it is Onderwerg in Nederland (ODiN) [77] or Mobiliteitspanel Nederland (MPN) [78]. The Motorized Transport in Germany 2010 (KiD 2010) survey is another German national transport-related survey that focused on trips made by motorized commercial and private vehicles, that even includes a question on parking location [79]. The analysis of this paper refers only to the survey presented in Table 2.

Table 2. List of NTSs conducted in an OECD country by continent and population.

Country	National Travel Survey	References	Question on Parking
AFRICA
South Africa	National Household Travel Survey 2020	[80]	N
ASIA
Japan	Nationwide Person Trip Survey 2015	[81]	N
Korea	National Transport Surveys 2021	[82]	N
Israel *	Israel Metropolitan Travel Habits Surveys (2010–2019)	[83]	Y
EUROPE
Germany	Mobilität in Deutschland (MiD 2017)	[84]	Y
France	Mobilité des personne 2018–2019	[85]	Y
United Kingdom **	National Travel Survey 2022	[58]	Y
Italy	Osservatorio “Audimob” sulla mobilità degli italiani 2023	[86]	N
Spain	Movilia 2006/2007	[87]	Y
Netherlands ***	Mobiliteitspanel Nederland (MPN 2022)	[78,88]	Y
Belgium	Belgium Daily Mobility (BELDAM) 2010	[89]	Y
Sweden	Resvanor i Sverige 2022	[90]	Y
Austria	Österreich unterwegs 2013/2014	[91]	Y
Switzerland	Mikrozensus Mobilität und Verkehr 2021	[92]	Y
Denmark	Transportvaneundersøgelsen 2019	[93]	Y
Finland	Valtakunnallinen henkilöliikennetutkimus 2021	[94]	N
Norway	The Norwegian National Travel Survey 2018/19	[95]	Y
Ireland	National Household Travel Survey 2022	[66]	Y
Croatia	Traffic Habits Survey 2017	[96]	N
Slovenia	Passenger Daily Mobility, 2021	[97]	N
Latvia	Mobility Survey of Latvia Population in 2021	[98]	N
Estonia	Survey on the mobility of Estonian residents 2021	[99]	Y
NORTH AND CENTRAL AMERICA
USA	National Household Travel Survey 2022	[100]	N
Canada ****	National Travel Survey	[63]	N
OCEANIA
New Zealand	New Zealand Household Travel Survey 2021–2022	[101]	Y

* Combination of multiple regional travel surveys; ** Covers just England; *** Questionnaire of the 2018 iteration (variable coding is identical in 2018 and 2022); **** Addresses the topic of touristic travel.

Table 2. List of NTSs conducted in an OECD country by continent and population.

Country	National Travel Survey	References	Question on Parking
AFRICA
South Africa	National Household Travel Survey 2020	[80]	N
ASIA
Japan	Nationwide Person Trip Survey 2015	[81]	N
Korea	National Transport Surveys 2021	[82]	N
Israel *	Israel Metropolitan Travel Habits Surveys (2010–2019)	[83]	Y
EUROPE
Germany	Mobilität in Deutschland (MiD 2017)	[84]	Y
France	Mobilité des personne 2018–2019	[85]	Y
United Kingdom **	National Travel Survey 2022	[58]	Y
Italy	Osservatorio “Audimob” sulla mobilità degli italiani 2023	[86]	N
Spain	Movilia 2006/2007	[87]	Y
Netherlands ***	Mobiliteitspanel Nederland (MPN 2022)	[78,88]	Y
Belgium	Belgium Daily Mobility (BELDAM) 2010	[89]	Y
Sweden	Resvanor i Sverige 2022	[90]	Y
Austria	Österreich unterwegs 2013/2014	[91]	Y
Switzerland	Mikrozensus Mobilität und Verkehr 2021	[92]	Y
Denmark	Transportvaneundersøgelsen 2019	[93]	Y
Finland	Valtakunnallinen henkilöliikennetutkimus 2021	[94]	N
Norway	The Norwegian National Travel Survey 2018/19	[95]	Y
Ireland	National Household Travel Survey 2022	[66]	Y
Croatia	Traffic Habits Survey 2017	[96]	N
Slovenia	Passenger Daily Mobility, 2021	[97]	N
Latvia	Mobility Survey of Latvia Population in 2021	[98]	N
Estonia	Survey on the mobility of Estonian residents 2021	[99]	Y
NORTH AND CENTRAL AMERICA
USA	National Household Travel Survey 2022	[100]	N
Canada ****	National Travel Survey	[63]	N
OCEANIA
New Zealand	New Zealand Household Travel Survey 2021–2022	[101]	Y

* Combination of multiple regional travel surveys; ** Covers just England; *** Questionnaire of the 2018 iteration (variable coding is identical in 2018 and 2022); **** Addresses the topic of touristic travel.

Table 3. Phrasing and response options of residential parking-location-related questions.

Country	Question	Respond Options
ASIA
Israel	VEHICLE related variable: Home Parking	(1) private parking, (2) on the street, (3) in the field, (4) unknown
EUROPE
Germany	Where is the VEHICLE usually parked at your home?	(1) on a public road, (2) parking garage/underground garage, (3) garage/carport/parking space on private property, (4) other
France	Where is this VEHICLE usually parked overnight? Where is this VEHICLE usually parked on a day when you are not using it? (Additional answer options)	(1) in a covered spot, garage, or box, (2) residents parking space (outdoor, backyard), non-residents parking lot (outdoor), (5) on a public road free of charge, (6) on a public road (fee)
United Kingdom (England)	Where is the VEHICLE usually parked overnight? Does the house/bungalow have a garage or car port?	(1) in the garage (at this address), (2) not garaged but still on the property of this address, (3) on the street or public highway, (4) or elsewhere (at or near your home) (5) does not usually park at/near home (1) yes, (2) no, (3) not sure
Spain	Availability of a garage parking spot? (HOUSEHOLD)	(1) none, (2) one, (3) more than one
Netherlands	Which of the following parking provisions are present in your neighborhood? (HOUSEHOLD)	(1) parking on your own property/driveway/garage, (2) public (free) parking/parking area, (3) permit parking, (4) paid parking
Belgium	Where do I most often place this VEHICLE at night?	in a garage, box, in another reserved place, which is located approximately minutes from my dwelling/on the street/other
Sweden	Where do you usually park your car when you are at home? (PERSON)	(1) parking for which you do not pay, (2) parking for which you pay
Austria	Parking spot at home in a garage/on private property (VEHICLE)	Y/N
Switzerland	How many parking spaces or garages do you have at home, at your main residence, whether owned or rented? (HOUSEHOLD)	number of parking spaces
Denmark	How are the parking conditions at your home? (PERSON)	(1) carport, garage on own property, (2) front yard/driveway on own property, (3) parking space on/near the property: reserved with number plate on sign, (4) only on the street/road, + additional elements
Norway	Do you have your own car parking space close to where you live? Do you have separate parking spaces for both/all in the vicinity of the property? How many cars do you have parking space for?	Y/N Y/N
Ireland	Where are your HOUSEHOLD vehicles normally parked when the driver is at this address? Tick all that apply	(1) on street (free), (2) on street (with a parking permit), (3) in driveway, (4) in garage, (5) in a parking bay, (6) in a car park, (7) other, (8) no cars/vans in household
Estonia	Where is/are the family car(s) usually parked at night? (HOUSEHOLD)	(1) in a confined yard area or in the garage, (2) in a public pay parking or on the street for a fee, (3) in a free public car park or on the street for free, (4) elsewhere
OCEANIA
New Zealand	Where did you park? (PERSON/ACTIVITY)	(A) off street, (1) residential property, (2) work/employer’s car park, (3) paid car park, (4) customer car park (unpaid), (5) public unmetered parking, (B) on street, (6) time limit, (7) no time limit, (C) did not park, (D) other (specify)

Table 3. Phrasing and response options of residential parking-location-related questions.

Country	Question	Respond Options
ASIA
Israel	VEHICLE related variable: Home Parking	(1) private parking, (2) on the street, (3) in the field, (4) unknown
EUROPE
Germany	Where is the VEHICLE usually parked at your home?	(1) on a public road, (2) parking garage/underground garage, (3) garage/carport/parking space on private property, (4) other
France	Where is this VEHICLE usually parked overnight? Where is this VEHICLE usually parked on a day when you are not using it? (Additional answer options)	(1) in a covered spot, garage, or box, (2) residents parking space (outdoor, backyard), non-residents parking lot (outdoor), (5) on a public road free of charge, (6) on a public road (fee)
United Kingdom (England)	Where is the VEHICLE usually parked overnight? Does the house/bungalow have a garage or car port?	(1) in the garage (at this address), (2) not garaged but still on the property of this address, (3) on the street or public highway, (4) or elsewhere (at or near your home) (5) does not usually park at/near home (1) yes, (2) no, (3) not sure
Spain	Availability of a garage parking spot? (HOUSEHOLD)	(1) none, (2) one, (3) more than one
Netherlands	Which of the following parking provisions are present in your neighborhood? (HOUSEHOLD)	(1) parking on your own property/driveway/garage, (2) public (free) parking/parking area, (3) permit parking, (4) paid parking
Belgium	Where do I most often place this VEHICLE at night?	in a garage, box, in another reserved place, which is located approximately minutes from my dwelling/on the street/other
Sweden	Where do you usually park your car when you are at home? (PERSON)	(1) parking for which you do not pay, (2) parking for which you pay
Austria	Parking spot at home in a garage/on private property (VEHICLE)	Y/N
Switzerland	How many parking spaces or garages do you have at home, at your main residence, whether owned or rented? (HOUSEHOLD)	number of parking spaces
Denmark	How are the parking conditions at your home? (PERSON)	(1) carport, garage on own property, (2) front yard/driveway on own property, (3) parking space on/near the property: reserved with number plate on sign, (4) only on the street/road, + additional elements
Norway	Do you have your own car parking space close to where you live? Do you have separate parking spaces for both/all in the vicinity of the property? How many cars do you have parking space for?	Y/N Y/N
Ireland	Where are your HOUSEHOLD vehicles normally parked when the driver is at this address? Tick all that apply	(1) on street (free), (2) on street (with a parking permit), (3) in driveway, (4) in garage, (5) in a parking bay, (6) in a car park, (7) other, (8) no cars/vans in household
Estonia	Where is/are the family car(s) usually parked at night? (HOUSEHOLD)	(1) in a confined yard area or in the garage, (2) in a public pay parking or on the street for a fee, (3) in a free public car park or on the street for free, (4) elsewhere
OCEANIA
New Zealand	Where did you park? (PERSON/ACTIVITY)	(A) off street, (1) residential property, (2) work/employer’s car park, (3) paid car park, (4) customer car park (unpaid), (5) public unmetered parking, (B) on street, (6) time limit, (7) no time limit, (C) did not park, (D) other (specify)

4.1.1. Questions on Parking Location

In 15 NTSs, there is typically only one question about parking locations, which can be distinguished between questions regarding availability or the usual use of parking locations. A special case is New Zealand’s NTS, where the parking location is documented in the trip diary after each trip. In most cases, the question relates to the situation at home. Occasionally, for example in England, there are additional questions about the workplace. In Slovenia, only the usual parking spot at work is queried. The questions and response options that could be found in the corresponding questionnaires are listed in Table 3.

In nine surveys, the parking question refers to the usual parking location. In addition to the phrase “at home”, the terms “overnight” or “at night” are also used. These terms primarily refer to home parking, as suggested by the corresponding answer options. The question is asked in multiple ways, most commonly as a single-choice question. In this category the reference level is either individual vehicles (Germany, France, England, Belgium, Israel), households (Ireland, Estonia), or persons (Sweden, Denmark). In Ireland, responding households are instructed to indicate all options that apply to any household vehicle when parked at home. In Denmark, respondents are asked about the parking conditions at home. The instructions specify that respondents should select the most frequently used option.

In five surveys, the availability or number of parking options at home is the only question about parking. In Austria and Norway, the question is dichotomous. In Switzerland, respondents are required to fill in the number of parking spots, while the Spanish survey offers predefined numbers to choose from. In some surveys, such as England and Germany, there are multiple variables about parking. The availability of a garage or carport at the household level is either surveyed (England) or estimated based on other information (Germany).

The depth of consideration given to different options varies according to the survey in question. In some cases, additional aspects such as fees are included. Questions about availability are usually less detailed. For example, in Austria and Switzerland, questions are asked about parking spaces and garages at home. In Spain, the question is limited to the availability of a garage parking spot. In contrast, in Norway, the question is much broader, inquiring about an owned parking space in the vicinity of the property. Due to the activity-oriented questioning in New Zealand, there are also response options for other locations such as workplaces. Due to different question phrasing, given response options vary. However, even for questions that are similar in their wording, the response options also can differ. An extreme case is in Sweden, where the question is phrased similarly to those in the German or English questionnaire, but the response options only differentiate between free and paid and do not include further information about the parking location as is the case in the other surveys.

4.1.2. Parking and Charging of Electric Vehicles

In a small number of surveys, respondents are also asked about the availability of electric vehicle (EV) charging facilities at parking locations. In France, the EV charging availability at the given parking spot was asked and if available, whether the charging was via ordinary plug or dedicated charging infrastructure. In Italy, the survey does not directly address parking, but instead inquires about the potential for EV charging. In this case, the response options include “Yes, I own a garage/car parking space with electric socket” and “No”. However, the latter option does not differentiate between specific reasons (e.g., owning no private parking space at all or owning private space but without electric socket) for a negative answer.

4.2. OECD Affiliated Countries Without a NTS

4.2.1. Regional Travel Surveys

During the search for NTSs, a number of regional or local travel surveys were identified. Exemplary regional surveys from Australia, Argentina, Peru, Portugal, China, and the USA were looked at. Some of the surveys inquire about the parking locations after each trip with great specificity [102,103]. However, some studies do not address parking at all [104,105], or tend to ask about parking costs [106]. In the United States, the National Household Travel Survey (NHTS) does not cover residential parking locations but parking costs. But such questions do exist in some Regional Travel Surveys, e.g., New York [107] and California [108]. These surveys ask where the vehicle is parked at the destination of a trip. Overall, the few regional travel surveys that have been analyzed follow a similar pattern to the NTS. It cannot be concluded that the issue of parking is more prevalent in this context. Nevertheless, depending on the area under consideration or the lack of NTS, such surveys can be potential data sources.

4.2.2. Housing Surveys

While none of the Latin American countries analyzed have an NTS, each of these countries has a Housing Survey. Argentina [42], Chile [40], Colombia [46], and Mexico [109] all inquire about residential car parking, while Brazil [110], Costa Rica [46], and Peru [111] do not. In Canada, the survey just inquired about inclusion of parking in the rent and not about the location [112]. In Asia, India [41] and Türkye [113], both without an NTS, conduct such a survey. While in India the focus was more on drinking water and sanitation, a question on parking was included in the Turkish survey. In accordance with the survey context, the questions pertain to dwellings or buildings. In contrast to the NTS, which frequently focuses on the availability of parking, there is no information available on actual usage, particularly when the presence of a vehicle is not inquired about. This makes it challenging to compare with utilization data from the NTS, where the parking location of each vehicle is frequently inquired about.

5. Discussion

It is evident that there are a multitude of different questions and, in particular, combinations of response options. This leads to different classifications of parking possibilities. Categorizing them based on the previously defined layer (Figure 1) is often only partially possible. Given the considerable number of potential options and limitations in the scope of survey questions, it was expected that not all layers could be fully differentiated. However, it is somewhat surprising that in several of the analyzed surveys, options differing in location, type, and access (private vs. public) were grouped together. In some cases, it appears that the respondents are left to their own judgement to interpret and differentiate certain options available for answering a question. In the questionnaires and technical, methodological, or results reports, there is no clear definition of the response options in question.

5.1. Precision of Phrasing—Example of the German and English NTS

The precision of phrasing in the native language may have been partially lost when using Google Translate and DeepL. However, the discussed patterns also appear in surveys conducted in both English and German. To clarify this, the differences and their impacts will first be exemplarily discussed using surveys from Germany and England. Subsequently, the surveys will be considered more generally concerning the informational content for various parking distinction criteria.

As identified in the results section, the questions in the German and English surveys are largely comparable. In both cases, a comparable formulation regarding the parking location on the street was chosen (on a public road/on the street or public highway). However, there are significant differences in the off-street parking options. In Germany, the response option “garage/carport and parking space on private property” covers both of the English options “garage (at this address)” and “not garaged but still at this address.” This distinction implies that in England, the exact location is limited to one’s own property, whereas in Germany, it is only specified as private property in general. With regard to the remaining response options, it is particularly challenging to categorize a parking space in a house’s underground garage in Germany. This parking space could be classified under the option “garage/carport and parking space on private property” or the option “parking garage/underground garage.” In England, such an underground parking space could be captured via the “garage (at this address)” option. In this case, a parking space in a nearby parking garage might not be covered. For this reason, there is the option to choose “elsewhere (at or near your home).” The option of “not parking at the residence” is only available in England due to the slight variation in the questions’ formulation.

Both surveys adopt a vehicle-centric perspective when considering parking. Consequently, the location of a specified number of household vehicles is queried in the survey. Additionally, both surveys include a variable regarding the presence of garages.

In Germany, only the proportion of households that own a garage but do not park any of their vehicles on private property can be determined. The proportion of garages that are not in use cannot be determined, as vehicles parked in front of the garage are included in the same response category as vehicles in the garage. In England, by separating garage and other options, it is possible to estimate how many households own garages but do not primarily use them for parking vehicles.

This comparison illustrates that questions and the provided answer options are not always unambiguous and vary between different surveys. The subsequent section will examine the various responses to the question, “On which level are parking spaces surveyed?” based on the layers presented in the methodology section.

5.2. Consideration of Different Layers of Differentiation

With regard to layer 0 (location), it can be stated that the residential parking location was frequently inquired about and is clearly distinguishable from other locations. This is consistent with other surveyed aspects that mainly illuminate the living situation and the surrounding area of the residence.

Due to the nature of the questions asked, it is not possible to derive a distribution of parked vehicles in some surveys even at layer I (off-street vs. on-street). In just 8 out of 15 NTSs, a categorization of vehicles at the first level is possible since on-street parking is listed as a separate option. In one survey, on-street and off-street options are combined. Surveys with questions considering availability usually limit the informative value to one type of off-street parking. A negative response does not automatically imply the reliance on on-street parking as there are multiple other forms of off-street parking. Overall, this layer offers the highest degree of comparability between the different surveys. After the first layer, a complete or clear classification is usually no longer possible. Even categories within a single survey are often not clearly defined and can be interpreted in different ways by the respondents.

The distinction between private and semi-public (layer II) is often not included in the surveys at hand. Public parking options are often not listed separately. In some cases, only specific options, such as non-resident parking lots, but not parking garages, are included. The categorization between private and public might not always be unambiguous. For example, there could be private parking spots in a publicly accessible parking garage. It may depend on whether the structure as a whole or individual parking spaces are being considered. In this context, the question arises as to whether a parking space in a parking garage that is physically separated from a building but accessible only to residents should be considered the same as a private parking space that is marked “Reserved with number plate on sign” (Danish survey) and is located in a publicly accessible parking garage or in a parking lot. This is related to the distinction regarding the exact location, whether the parking facility is on or off the property.

The distinction between on- and off-property (layer III) is handled heterogeneously. Sometimes off-property options are included without further differentiation, while at other times the focus is solely on the individual’s property. The questions posed also frequently allow for a degree of interpretation. For instance, does “at home” refer solely to one’s own property, or does it also encompass the surrounding area? Furthermore, a classification at this level is particularly susceptible to differing judgment of options at the fourth layer.

At layer IV, the parking options are divided into different types of infrastructure. In multiple surveys, it is not clear whether the answer option “garage” also includes parking spaces in an underground garage located on the property. If this is the case, there would in most surveys be little distinction between single garages and underground garages for multiple vehicles from different households. If this is not the case, the option “parking garage” would not differentiate between private and public, nor between on-property and off-property. In general, the merging of certain parking options is carried out in different ways. Garage and driveway are sometimes grouped together, but often they are also differentiated. In some cases, only garage spaces are of relevance.

5.3. Differentiation by Reference Level and Potential Statements

In addition to the consideration of layers, the reference levels and the question of whether availability or utilization of the parking facility is considered also differ. Availability and usage are not asked in combination. There are sometimes questions about the availability of garages in surveys asking about the usual parking spot, but it is rarely possible to combine this information of both questions, as garage parking spots and other options on private property are often grouped together. This leads us to question how well the “usual” parking location reflects the availability of private parking options. Studies, such as those by Scheiner et al., Taylor, and Volker and Thigpen [26,27,29], indicate that private parking options are not utilized to their full potential, even in high-density areas with limited amounts of public street parking.

Besides New Zealand, no other travel survey uses activities as references. The advantage of inquiring about the parking location at the activity level is that the specific parking location is recorded after each trip. However, this approach is deficient in that it lacks information about vehicles that were not moved during the reporting period. Furthermore, it increases the burden on respondents, as this question is part of the trip diary and must be answered for every car journey.

5.4. Relevance of Residential Parking in National Travel Surveys

Due to its structure and trip diaries as a key element, NTSs primarily target the movement of the population. While parking is involved at the beginning and end of car trips, it is considered less significant than the number of trips or the purpose of trips. A survey of 81 experts regarding data needs indicated that characteristics of the dwelling, household vehicles, and parking facilities are seen as less essential [114]. This is also evident from the fact that parking is often only briefly mentioned or not mentioned at all in the final reports of the considered surveys. On the other hand, the authors of the study KiD 2010 state that the question on the type of parking space at the end of the journey was newly included for the 2010 survey in order to answer questions in the field of transport planning and for the research field of electromobility [79].

The topic of parking is missing entirely or is not considered absolutely necessary in guidelines or specifications for the harmonization of surveys at EU level. At least “car-parking possibilities at home or workplace” are listed as potential national additions in recommendations. Other variables also not seen as a necessity are household size and composition, dwelling type, and income, which might be used to explain differences in the availability of private parking spots [65]. Not being part of such recommendations might pose difficulties in addressing the topic of residential parking in potential new NTSs.

5.5. Suitability of the Surveys Considered in Relation to Residential Parking

The suitability of a given survey question for the analysis of private parking depends on the specific research question and the additional variables present. If information on the number of cars and available parking spaces is available at the household level, this provides a comprehensive overview of the total inventory of private residential off-street parking on building property. If the information is available at the vehicle level, it is also possible, depending on the data structure, to consider the parking options utilized by a household.

One potential use case arises in the area of vehicle electrification. As stated in the introduction, private parking availability plays a crucial role in the distribution of charging infrastructure. In terms of the potential development of charging infrastructure, it is less important where the vehicle is usually parked, but rather whether a private parking space is available. Whether the question refers to vehicles, individuals, or households is less relevant. All vehicles in the household could be considered adequately provided for if at least one vehicle or the household as a whole has at least one private parking space, assuming the possibility of alternating charging. The potential to install a wall box and charge privately might even incentivize people to use the private parking space available to them, improving utilization and relieving public space.

The distinction on specific levels is of varying importance. In the case of single-family houses in low-density neighborhoods, differentiation between on- and off-street parking is usually sufficient. However, the situation is different for multi-family houses, where the specific parking situation strongly influences the possibilities and potential for constructing charging infrastructure. In underground garages associated with buildings, with a small group of authorized access, there are still hurdles to overcome. Nevertheless, in some countries, such as Germany, there is at least a legal right to install charging infrastructure. In parking garages, it is probable that the process will be more challenging and that a centralized approach will be necessary, with specific responsibilities being designated. In public parking spaces or in public parking garages, the installation of private charging infrastructure for individual vehicles is less likely to be realized. In such cases, publicly accessible charging infrastructure, which tends to have higher prices and lower availability, is more probable.

6. Conclusions

The issue of parking does not always receive the necessary attention. This is shown by the fact that of the 24 NTSs from the 49 states surveyed, only 15 included a question on parking. Most NTSs were found in Europe, where residential parking locations are frequently addressed. No NTSs were found in South America, or in the populous countries of India and China.

When comparing various NTS, the comparability of statements depends on the design of the questions and response options. Typically, there is only one question related to parking location. Sometimes the questions are similar, but the response options vary significantly. Multiple surveys only allow an unambiguous differentiation on the level of on-street or off-street. In other cases, it is not always clear which locations are meant by the different response options. Often, it is unclear whether the parking is private or semi-public and whether it is on the property or elsewhere. Sometimes, conclusions about off-street parking as a whole cannot be drawn, only about individual sub-locations. Follow-up questions for further detail, such as charging options for electric vehicles, are rare.

To conclude, questioning of residential parking spots can be carried out in multiple ways. The question itself is most of the time straightforward, but the chosen answer options often leave room for interpretation. This makes it difficult to accurately assign vehicles to parking locations. In addition, using a parking space is not the same as having a parking space. It is therefore important not to issue simplified statements based on these surveys. However, in doing so, this might lead to less concise and more complicated statements.

Varying degrees of specificity and differences in survey design make it challenging to use NTS data uniformly for analyzing residential parking availability. However, multiple NTS provide sufficient detail to allow for national analyses of parking facilities, but discrepancies in question phrasing hinder direct international comparisons.

As most NTS questionnaires tend to already contain a large number of questions, it is not recommended to increase the information content on the topic of parking locations by adding several new questions. Rather, it should be ensured that the question already included is formulated accordingly. In particular, the response options should be phrased unambiguously and clearly differentiated from each other. In order to maintain comparability with previous iterations, it should ideally be possible to aggregate the new options to the old response options. To ensure a common understanding, additional information or definition for each response option would be helpful for both respondents and researchers working with the data. In order to measure both usage and availability, it is advisable to combine a question on the availability at household level with the actual usage at vehicle level.

As with any change to established surveys, practical barriers such as stakeholder resistance and the financial and logistical costs of revising established survey protocols must be carefully considered. However, a review of changes between survey iterations also shows that adjustments are often made, sometimes significantly more than changing the answer options of a question. In addition, it has been shown that the parking variable is often not the focus of attention, so changes are unlikely to encounter the kind of resistance that would occur with central elements such as the travel diary.

Depending on the country, different data sources, such as housing surveys or censuses could be used instead or in addition to NTSs. In particular, the difference between availability and actual use can be examined more closely. This applies in particular if the data sources have comparable spatial units.

In addition to the question about parking, it is also relevant which other things are queried, can be combined with parking, or can explain differences in the availability or use of certain parking locations in different spatial arrangements. It should also be explored whether data sources from other ministries less related to construction and transport, such as tax information, could be made available to researchers to some extent.

Web-based searching for the relevant surveys had its difficulties. Surveys that meet the criteria are often not called “National Travel Survey”. Technical reports and questionnaires were much harder to find than reports on results and are often only available in the respective national languages. In the context of considering various regions and countries in international research, it is advisable to establish and encourage naming standards, such as including the keyword “National Travel Survey” in English in addition to the name in the given national language.

The reliance on translators/dictionaries to translate various reports is one of the main limitations of this study. If documents were not available in English or German, it is possible that these have either been overlooked or that the precision of certain terms has been lost. Furthermore, the dissemination of different NTSs consists of varying numbers of reports, codebooks, appendices, and data. This increases the risk that relevant information is distributed in different locations and was not always fully considered. While our study discusses the potential effects of different response options in relation to actual parking location, it is unclear how they actually affect data quality and bias.

Future research could include conducting experimental studies to assess the effects of specific question wording and response options on responses and data outcomes. More thematically orientated further research could take a closer look at individual NTSs with similar questions and answer options and evaluate the extent to which various aspects such as community type, building type, public transport accessibility, and number of vehicles per household affect the available parking options.