Urban areas are becoming the main target of climate change [1
]. The effects of global warming and climate change are expected to be exaggerated in cities, particularly with respect to extreme heatwave events [2
]. The phenomenon of urban climate must be comprehended in the local and regional contexts, in which all dynamics and impacts are undertaken in relation with the expansion of urban sprawls. Both the diminishing of green urban areas and the increase of artificial surfaces are aggravating the impact of heatwaves (HW). The greatest HW temperature increases are expected in Central European cities, whereas cities such as Ljubljana, Prague, and Zagreb will experience a significant rise in HW [4
]. A number of studies, including observational, modelling, or both, focusing on the micro to the meso-scale, have investigated the effectiveness of using different mitigation strategies to reduce the urban heat island (UHI) in various cities. Strategies aimed at increasing urban vegetation in urban areas are referred to as green infrastructure (GI) strategies [6
Urban areas are characterized by a large proportion of artificial surfaces, such as concrete and asphalt, which absorb and store more heat than natural vegetation, leading to the UHI effect [11
]. Measures to reduce UHI usually target the improvement of thermal comfort in cities [3
], particularly in residential areas [6
], and involve extensive greening schemes. Here, changes in traffic flow play a special role by introducing measures to encourage sustainable urban mobility. These include putting initiatives in place to reduce the number of vehicles in city centers by introducing park-and-ride (P + R) parking facilities on city outskirts. P + R is a system of nodes, which spatially, organizationally, and often also in pricing, combines parking areas with public passenger transport stops. They are intended for parking personal vehicles on the outskirts of the city or as close as possible to the origin and transferring to public transport (bus or train), taking the passengers to the city or city hubs [17
]. Parking organization in large parking lots along major arterial roads and the use of public transport and other forms of sustainable mobility facilitates the arrival to work, while also improving the air in the center and reducing the needs for parking areas in the city. On the other hand, the concentration of cars in parking areas that (mostly) lack greenery brings the UHI effect [18
] to the city outskirts as well.
This study does not address the implications of extensive parking areas on micro locations, but, rather, it focuses on the problem of high temperatures in the vehicles parked in parking areas exposed to sunlight during summer when people return to their cars after work and drive home. Owing to the evolution of technologies, we have become increasingly used to a balanced climatic comfort, which increases the sense of living discomfort outdoors. Increased ambience temperatures deteriorate the physical well-being of people causing problems, such as “heat stress in the form of heat syncope, thermal exhaustion, cardiovascular stress, cardiorespiratory diseases and heat stroke“ [19
]. At extreme air temperatures, humans are at higher risk of, for example, heart attacks and asthma, thereby increasing morbidity rates among vulnerable population groups [21
The greenhouse effect leads to a temperature rise in the vehicle even at relatively low temperatures of outside air and little clear sky [22
]. The latter is due to the transmission spectrum of glass, which transmits the sun’s radiation of shorter wavelengths. Sunlight, therefore, heats up the incident surfaces inside the vehicle, which emit long wave thermal radiation due to the increased temperature. This radiation is not transmitted by the glass, leaving heat “trapped” inside the vehicle. McLaren et al. [23
] measured the temperature rise inside a vehicle on 16 sunny days (with ambient temperatures ranging from 22 °C to 36 °C). Even on a cooler day with an outside ambient temperature of 22 °C, the internal temperatures reached 47 °C. Marty et al. [24
] measured temperatures in vehicles under varying weather conditions and in various seasons and found that when cars were exposed to sunshine even during winter, the interior temperature rose up to 30 °C, and in spring and autumn, up to 60 °C, while in summer, interior temperature extremes of up to 90 °C were recorded.
A parked vehicle typically heats up very quickly, which first affects the thermal comfort of the driver and passengers. However, even more dangerous than the personal thermal discomfort is the correlation between high temperatures and a lower alertness of the driver, while consequences can be fatal. Driving demands a high level of concentration from a driver; therefore, optimal physical conditions must be met. Heat stress can produce detrimental effects on motor response and some cognitive deficiencies may be attributed to decreased motor performance [25
]. Mental task is differentially sensitive to thermal stress [26
] and reaction times are longer for hyperthermic subjects [27
]. Based on the measured temperatures and humidity [25
], with increased temperatures, drivers’ performance losses of up to 50% are predicted even for relatively simple tasks, such as keeping the vehicle on a straight course. Performance losses in excess of 75% are predicted under the most extreme thermal conditions for demanding tasks, such as correctly identifying a signal and reacting in due time [25
Studies have found compelling evidence of lower alertness, implying a higher chance of missing signals, and slower responsiveness, implying longer reaction times and higher accident rates. In the study by Wyon et al. [28
], the alertness and driving performance of drivers driving for one hour on public roads was measured (in sections with speed limitations of 50, 70, 90, and 110 km/h); the drivers were randomly assigned to one of two thermal conditions (21 °C or 27 °C). They found that the negative effect of heat stress on vigilance was statistically significant. At 27 °C, the overall proportion of missed signals was 50% higher and response times were 22% longer than they were at 21 °C. These effects of heat were significant and proportionally greater in the second half-hour for subjects <40 years and for speeds below 60 km/h (i.e., in city traffic). The latter finding suggests that heat may have increased arousal, and there was some indication of a redistribution of attention away from the most peripheral signals at the higher temperature.
Urban greenery can help to regulate urban microclimates. Green areas are actually the most efficient element that help to reduce the UHI effect [28
]. Adding more urban trees, parks, gardens, wetlands, and green roofs within urban areas is generally referred to as the implementation of GI strategies. Trees mitigate air overheating, offer shade, and increase the feeling of humidity with their breathing leaves. Vegetation is the necessary element of parking areas, with trees and green areas in between the parking places, as only this can prevent the overheating of concrete, asphalt, and metal surfaces to intolerable temperatures. Importantly, the specificity of P + R sites is the following: the car is parked at the P + R site for several hours, while we return to it from a hard day’s work from overheated spaces. In any case, the city is a much friendlier urban place if high-quality green mobility is taken care of. Increasing the vegetation land cover could considerably reduce surface temperatures at parking lots [20
]; trees of a height of 5–10 m help to control the overheating of surfaces [18
]. Gillner et al. [31
] recommend decreasing the thermal load in urban areas for future tree planting by choosing species with high cooling potential. The location of trees at a parking lot has an influence on solar exposure reduction. Bajsanski et al. [18
] developed and demonstrated an algorithm that optimizes the location of trees, aiming to provide the maximum overshadowing of parking lots.
The main purpose of this paper was to underline the problem of the rate of temperature rise in parked vehicles in large parking areas exposed to sunshine and propose solutions with an optimal introduction of vegetation as protection against the sun. The research of the relation between built and open (void) spaces refers to the effects of surface and atmospheric UHI. The case study of Ljubljana in Slovenia offers an opportunity to explore spatial and microclimate qualities referring to sustainable mobility systems. Besides the pioneering exploration of the aforementioned conditions at open P + R lots in the city of Ljubljana, the novelty of the presented study is the research methodology, which represents a combination of several types of methods, from ex ante problem observation, to experimental work in situ, to computer simulations and modeling, to an ex post multicriteria comparative analysis of the obtained results.
Further evaluation and optimization of the developed models can be done on the basis of the “aesthetics,” “amenity,” and “shading benefits” that were explained by Nieuwenhuijsen [46
] in the context of urban and transport planning for livable and healthy cities as elements that “have also been shown to provide significant economic benefits, while benefits in terms of water regulation, carbon reduction and air quality are usually more modest” [47
To assess the visual quality of the derived variants, trees and living walls on the parking lot and trees as a boundary condition should all be taken into consideration. Two main assessment principles can be established: the level of the visual perception of the parking lot from afar, and the level of the visual perception of the belonging parking system—i.e., the density of greening elements. Following these criteria, it can be concluded that densely green Model A and B (Figure 4
; Models A1 and B1) interrupt the visual continuity, but at the same time offer a pleasant feeling. Models C and D provide for the inclusion of the green wall element, which is of interest in terms of composition, but does not improve shading. Model C is approaching the values of shading Models A and B (Figure 4
—Models A5, B5, and C5); however, in terms of area perception, this implies restricted visual contacts. According to all the conditions investigated, Model D is the worst variant, even though the position of elements in the E–W direction allows for a visual connection when approaching the P + R (the access road is in the N–S direction). Even though the green wall is interesting, it is, in fact, less suitable in terms of maintenance. For discussion reasons, we added Model X, which offers tree densifications, creating the perception of green/forest islands. Model X does not follow the existing parking area design (the starting condition in the study was the existing design); nevertheless, it is important in the sense of comparing green areas and the number of trees. The tree concentration is suitable; however, the (non-functional) driving dynamics is put under question. Shading brings an added aesthetic value to a P + R facility, as it positively affects the perception of space under extreme heat. Models A and B offer the most shaded areas; from the point of view of a visual continuity, it would be recommended to remove the trees representing the boundary conditions of the site. Given the number of trees and considering all the criteria, Model B is the most rational one and gives an added value to the artistic organization of the green element.
In this study, we addressed the thermal comfort level related to the rise in temperatures inside vehicles parked at a P + R facility and the threats to health and drivers’ psychophysical performance. Moreover, it is important to understand the interplay of the results given in Figure 4
with temperatures and shading. The sense of ventilation, airiness, tree density, etc., are the elements that affect the comfort level at a P + R facility. It is necessary to explain that all these elements decrease the temperatures on the ground (as indicated by the models); however, trees are three times more beneficial than other green elements. The aforementioned elements and model depictions confirm the positive roles of Models A and B, but with restrictions referring to tree density and the visual significance of the P + R role, which is not a green area, per se.
Adequate green elements introduced in an open parking area not only provide shade and prevent the overheating of parked vehicles, but also contribute to the reduction of the outdoor air temperature due to evaporation. The temperature values of the areas shaded in vegetation were perceived as the most comfortable in Model C (Figure 4
, Model C5). Nevertheless, given the progression of shading, we find that the green wall as a shading element present in this, as well as in Model D, does not adequately support the P + R organization. Furthermore, shading by a green wall does not affect a person in the way that a cool tree canopy would (it is more like shading provided by a building). The most favorable natural shading element would be a pergola, which would cover the whole parking area and, thus, provide permanent shading. However, it is necessary to note that this decreases the significance of the elements presented in Section 3.1
and Section 3.2
of this paper.
Overall, given the primary function of an open parking area, this study demonstrated the need for optimization between the capacity and organization of a parking lot, on the one hand, and the type and spatial distribution of green elements that provide shading, on the other. In the existing parking area P + R Barje, the trees were identified as the most important element in providing shading in the variant when the shading is provided between parking spaces.
This study revealed the interrelations between the greenery in open parking areas, the temperature inside parked vehicles, and the local UHI effect. This study was conceptualized as two-stage research. The first (preliminary) stage dealt with the measurement and analysis of temperature data at two nearby open parking lots in the city of Ljubljana, one of which was the main research spatial area (P + R Barje, L1), and the other (Trnovo parking, L2) was used for comparison. The second phase of the study was dedicated to modelling the greenery at the principal location L1. Accordingly, the presentation of findings in this paper referred separately to each of the two stages. Progressive result generation allows for adjustments and, thus, enables a wider application of the designed research methodology.
Like most European capitals, Ljubljana is susceptible to the manifestation of UHI [15
]. While urban morphology initially influences the occurrence and intensity of UHI, ultimate consequences are felt by the users of the space. As demonstrated in this paper on the example of two nearby open parking locations in Ljubljana that were similar in all parameters except for the density of the greenery, the UHI effect is more intensive on the lot characterized by sealed artificial surface and scarce greenery (location L1), in spite of adjacent natural green areas. Likewise, the temperature in the cabin of the vehicle parked at location L1 was significantly higher in comparison with location L2, which had sufficient greenery to provide constant shade to parked vehicles.
Parking the vehicles in the open space during the warmer part of the year can easily result in the overheating of the cabin, especially when the duration of parking time is longer. The measurements showed that parking on an open surface without solar protection raises the temperature in the cabin to such an extent that it becomes dangerous for the health of the driver and other passengers in the vehicle, but also for traffic safety. For illustration, on the warmest day of measurement period, when outdoor air temperature was 35.5 °C, the temperature of the air in the vehicle cabin rose to 59.1 °C. High air temperatures in vehicles were measured also during moderately hot sunny days, which was also found by other authors [22
The key measure to prevent overheating in the cabin of the vehicle parked in an open space is shading. Though the shade can be produced by natural (e.g., trees) and man-made objects (e.g., the overhang) [21
], greenery offers far more benefits: inter alia, the reduction of the aforementioned UHI effect [6
]. Therefore, shading by greenery was adopted in this paper as the strategy aimed at improving the conditions by modelling different variants of greening parking lot L1.
Another important input to modelling was the fact that the vehicle must be parked in shade at all times, without (even short-term) exposure to direct sunlight. The desired state currently represents a challenge to P + R lots in many European cities. To that end, a transitional practical solution could concern the categorization of individual parking places according to the estimated length of parking time. In research terms, the continuation of the presented study could deal with the length of time during which the indoor air temperature of a vehicle parked in an open space rises above the recommended thresholds, examined against various shading-related conditions. An alternative research direction that would help to strengthen the validity of the obtained results refers to the in-depth analysis of drivers’ behavior at open P + R surfaces in terms of selecting a parking place, and the practice of precooling the vehicle cabin before leaving the parking area.