Taxis play an essential role in urban public transportation systems due to their speediness, convenience, efficiency, capacity to provide door-to-door service, privacy, flexibility, and 24-h availability. Since the average daily mileage of a taxi is usually higher than a private car, the importance of taxis and their influence on daily traffic, air pollution, and fuel consumption have attracted much attention. Recently, different approaches have been presented to reduce traffic congestion and air pollution, such as ridesharing, ride-splitting, and on-demand telephone taxis, along with the application of natural gas, hybrid, and electric vehicles [1
One of the most useful methods could be decreasing the taxi size, which has a positive effect on traffic efficiency, the capacity of transportation infrastructure, and a reduction in vehicle weight. In many cases, taxis are only occupied by one single-seat passenger, especially in commercial centers, where travelers often take a ride without baggage, such that the taxi does not need to use the trunk. Even in the case that some passengers carry baggage, the vehicle roof could be armed with a trunk to hold the baggage. Various studies in China have shown that the average taxi speed is usually around 30 km/h in populated urban areas [3
]. According to the vehicle’s speed versus drag force graph in the Automotive Aerodynamics Handbook [6
], which shows that the impact of aerodynamic drag force under low speeds (below 25 km/h) is small, the use of a trunk on the vehicle roof does not have severe negative impacts on the aerodynamic drag force. One advantage of decreasing the car size to a non-trunk hatchback car is a reduction in car weight, which positively reduces fuel consumption and air pollution, with consideration to the considerable daily mileage traveled of taxis. Also, the lower price for small cars and lower maintenance costs could be strong motivation for taxi drivers and fleet managers to downsize taxi vehicles.
In terms of safety, when the vehicle is small and has a lower weight, the safety of drivers, passengers, and pedestrians in an accident can be decreased, according to Newton’s law. In metropolises with high population densities and relatively low traffic speeds for taxis, the probability of accidental injuries to passengers can be decreased, because the kinetic energy will be lower for smaller cars. Also, automakers improve safety issues with side airbags and more residence chassis productions, and drivers have better maneuvering capability with small vehicles to avoid incidents [7
The definition of a small car in this study is a four-door hatchback, non-trunk, two-seat rows car, which is smaller than standard taxis by 80 cm in length and 15 cm in width. Figure 1
presents a schematic design of the two cars. The left image is a small car (according to some models of hatchback vehicles, such as Toyota Aygo, Citroen C1, BYD F0, Chevrolet spark, Peugeot 108, Skoda Citigo, Kia Morning, Mini Cooper, Geely Merrie, Hyundai I10, Fiat Panda, Volkswagen Up) and the right image is a standard taxi in Hangzhou, China (Hyundai Sonata).
In this research, we investigate the influence of replacing the current taxi vehicles with small cars through the following procedures: (a) field data are collected at nine locations representative of Hangzhou for investigating how many passengers use taxi trunks; (b) GPS data of over 7000 taxis are analyzed to reveal the travel patterns of taxi passengers, temporal profiles of taxi speeds, most popular taxi trips, and average daily travel mileage; and (c) we estimate and discuss the effects of taxi downsizing on fuel consumption, emission, safety, and financial issues.
The rest of the paper is organized as follows: Section 2
presents a literature review of the taxi GPS data usage, taxi fuel consumption, air pollution, and safety. Section 3
describes the taxi fleet in Hangzhou and shows the results of field data. Section 4
presents the impacts of taxi downsizing on fuel consumption, emission, safety, and economic efficiency. Section 5
presents a summary of the effects of taxi downsizing on the taxi industry and sustainability. Finally, Section 6
concludes the paper and outlooks for future research.
2. Literature Review
The role of taxis is significant as a part of urban public transportation systems. For instance, taxis account for 20% and 25% of the number of daily trips in Shanghai [8
] and in New York City [9
], respectively. According to the Government of Hong Kong [10
], more than one million people used taxis for their daily trips, with a total number of 15,000 taxis in the city. Another study by Yang et al. [11
] showed that 60% of trips in the central area of Hong Kong were completed by taxi. In this section, a literature review of relevant studies on transportation sustainability, taxis, taxi GPS data, and effects of urban taxis on the air pollution emission, fuel consumption, and safety is presented.
2.1. Transportation Sustainability
The World Commission on Environment and Development defines sustainable development as, “development which meets the needs of the present without compromising the ability of future generations to meet their own needs” [12
]. Sustainable development suggests a good balance between present lives and caring for future communities [13
]. Sustainability is commonly explored in terms of the theories of sustainable development, and public transportation is a major issue of urban sustainability, which has impacts on the economy, environment, and society [14
]. These three categories of issues are the primary considerations of sustainability, and sustainable transportation aims to meet the requirements to maintain sustainability [15
Transportation is a major source of air pollution [17
]. Vehicles emit greenhouse gases (GHG) such as CO2
, which have an impact on climate change and bring air pollution, which is harmful to public health [18
]. GHG and air pollution have influences on both the environment and society. Therefore, there is a need to find a way to solve these problems and make the transportation system sustainable. Fergusson [19
] pointed out that sustainable transportation should avoid damage to the environment. Cloutier et al. [20
] addressed that sustainable transportation even has impacts on the well-being of society. Transportation has significant effects on economic growth [21
]. Martin and Rogers presented that a transportation pattern developed the regional economy [22
]. Mosaberpanahi and Khales [23
] explained that the satisfaction of a community based on accessibility to a cheaper transportation system was a part of sustainable transportation.
2.2. Taxi GPS Data Analysis
GPS devices in taxis can be utilized to monitor travel patterns of passengers, e.g., movements and activities. Taxi traces show where passengers are picked up and dropped off, which routes are taken, and what steps the driver takes a cruise to find a new passenger. There are fruitful studies on the exploration of taxi GPS datasets in the literature. Some studies related to this paper are summarized as follows. Gühnemann et al. [24
] studied the relationship between pollution and vehicle speed in cities. In each route, they obtained the vehicle speed by using GPS data, then estimated the pollution in different areas. Chang et al. [25
] found out which areas in a city had high taxi demand according to the time, position, and weather conditions. Li et al. [26
] estimated the volume of passengers in various hotspot places and prepared a strategy for vacant taxis according to the historical taxi GPS trajectories. Fathi and Krumm [27
] illustrated the distribution of GPS paths near a location by a localized shape descriptor for searching road intersections automatically. Zhang et al. [28
] showed that it was possible to estimate travel time and speed by using GPS data. With taxi GPS data, it was known where passengers had been picked up and dropped off, which was very useful for detecting locations of high-ranking interest. Wang et al. [29
] analyzed the travel patterns of passengers by using hotspots of pick-up and drop-off locations. Liu et al. [8
] found that the land use of different areas (e.g., commercial, residential, and recreational land use) affected taxi pick-up and drop-off patterns. Luo et al. [30
] used the taxi GPS data in Shanghai Metropolis for determining the distribution of fuel consumption and emission, then provided a spatial–temporal map of emissions.
2.3. Fuel Consumption
As is well-known, a moving vehicle needs energy to overbear gravitational losses, aerodynamic force, and rolling resistance. An increase in mass requires that the engine burn more fuel. Decicco and Ross [31
] found that a 10% reduction in automobile mass with 1300 kg weight would gain a 6% saving for fuel. In another study, Burgess and Choi [32
] showed that car energy demand was intimately related to the car mass. They estimated that a 10% reduction in the mass of a car decreased the energy demand by 8%. Cheah and Heywood [33
] used computer simulation software to show that the fuel-saving would be 0.3–0.4 L/100 km if the car mass was decreased by 100 kg.
2.4. Air Pollution
In the literature, to the best knowledge of the authors, there was no fruitful work about taxis in selecting various fuels, powertrains, spare part expense, or carbon footprint impact. Gao and Kitirattragarn [34
] investigated the impact of taxi fleet emissions in New York City. The results showed that emissions were reduced by 2.29% for CO2
, 1.45% for CO, 1.12% for HC, and 1.7% for HO, by replacing 10% of taxis with hybrid-electric vehicles (HEVs). Another case study in Mexico City presented that the taxi fleet accounted for 6% of the daily trips, but it produced 13% of greenhouse gases (GHG) and 10% of NOx and CO of the road transportation [35
]. Wang et al. [36
] measured the whole vehicle kilometers traveled (VKT) in Shanghai and showed that taxi VKT was 18.2% and taxi emissions for CO and NOx were 22.2% and 10.4%, respectively. In recent research, Li et al. [37
] revealed the effect of taxi pollutant emissions in Nanjing. They suggested providing high-quality fuel and improving the taxi occupancy rate to help reduce taxi pollution.
The automobile mass and speed are two important factors of harm and fatality for passengers. Kahane [38
] showed that even a 50 kg reduction in car mass (but not truck mass) would improve safety, by decreasing the number of fatalities in other cars, pedestrians, bikers, and motorcyclists. Wenzel and Ross [39
] discovered that drivers had a high risk with car mass reduction. Tolouei and Titheridge [40
] considered the relationships of mass, fuel consumption, and safety. They expressed that car mass made a conflict between safety and fuel savings. The advantage of mass deduction was fuel saving but might change the crash risk. Nevertheless, there were a few studies on this matter which had inconsistencies with the results. Kahane [41
] presented that injury risk decreased with increasing the car mass. Their outcomes, based on data of 13 countries, demonstrated that there was no relationship between changes in these two different issues [42
]. Ross and Wenzel [43
] presented that even many sport utility cars (SUVs) were more dangerous for drivers in contrast with subcompact cars.
Regarding the impact of speed on safety, Buzman et al. [44
] realized that a 10% reduction of speed in the collision could decrease injuries and fatalities by 24% and 40%, respectively. Also, that World Health Organization [45
] reported that a driver could increase the crash risk of injury and fatality by 3% and 5%, respectively, by increasing their average speed by 1 km/h. Kloden et al. [46
] presented that if the speed was higher than 60 km/h, the accident risk would double with a speed change of 5 km/h.
There are many studies in the field of fuel consumption, vehicle mass, and air pollution, but the focus has usually been on private vehicles. In this paper, we study the effects of taxi downsizing on some issues, such as pollution, fuel consumption, safety, and economic efficiency, which are significant for urban transportation systems and sustainability. Taxis have high maintenance costs due to the high annual mileage and high initial investment cost. Renewing is required after a certain operational period (usually five years). Besides this, they usually move in city centers and regions with a high population density, vehicular pollution, and traffic jams. Thus, these considerations motivate us to understand the impacts of taxi downsizing on fuel consumption, emission, safety, and economic efficiency, and furthermore, to evaluate the impacts on transportation sustainability.
A method with high validity feedback for cognizing the influence of taxi downsizing on the aforementioned factors is naturalistic driving (ND). This method gives results based on driving under real conditions, with minimum external effects. In this case, we could choose two kinds of taxi, small taxis and standard taxis, and get the feedback after a certain time, so that the results would be reliable based on the real conditions. Balsa-Barreiro et al. [47
] worked and explained the basis of this method. For our study, this method would be costly, and we could not conduct the real test, so this would be worthwhile for future studies.
As shown in the results above, the benefits of taxi downsizing are clear and impossible to ignore. To understand how many taxis should be replaced with small cars, we need comprehensive spatio–temporal data of the demand for the standard taxi. If taxi downsizing is conducted for the taxi system in Hangzhou, and 50% of taxis are changed from standard taxis to small taxis (since the trunk usage in Hangzhou is only 12%, assuming that replacing 50% of taxis is acceptable), there will be significant benefits regarding the environment, economy, and society.
summarizes those benefits. The monetary saving in the purchase cost for replacing half of the taxi fleet from standard cars to small cars, in a city such as Hangzhou, is remarkable (about 15.75 million USD), so that the taxi service company can invest in providing better services. Also, we estimate the fuel cost difference would be 18,855,900 USD, which is a good point for drivers about the benefits of driving a small car instead of a standard car. Also, there are reduced costs in insurance, maintenance, and tax by 730, 787.5, and 1392.3 thousand USD, respectively. The estimation of 10 million liter fuel saving is another main benefit of taxi downsizing, which reduces annual emissions of CO, HC, and NOx by 1965, 98, and 58 ton, respectively.
Our findings show that most taxi seats and trunks are not occupied. Mitigating vacant taxi space creates some environmental, economic, and social benefits, which coincide with the sustainable transportation systems, as discussed by Loppolo et al. [77
]. Nowadays, sustainable development is a major concern of governments’ policies for the future of the world [78
]. Taxi downsizing can improve the efficiency of taxi services by reducing fuel consumption, emissions, raw materials, road space, road amortization, and taxi station space can be introduced as an environmentally friendly vehicle. The positive economic effect of taxi downsizing is clear, because a small taxi has cheaper expenses than a standard taxi (for a small car, the initial vehicle purchase cost, maintenance cost, depreciation cost, insurance cost, and tax cost are cheaper than a standard car). When a small car is used as a taxi, drivers can charge a cheaper fare based on the lower operating cost, so it is also beneficial for passengers. Small cars, due to the cheaper cost, can be a good option for carsharing services of transportation network companies (TNCs). Governments can provide subsidies for taxi drivers using small cars. The taxi downsizing also has inherent benefits, such as the reduced health cost due to the reduced pollutants. Other influences of small taxis in a city can be expressed in accidents. Pedestrians will have lighter injuries in the case of being hit by a small taxi in comparison with the standard car, although the passengers in small taxis are at a higher risk of severity in the accident. A solution to this could be preparing specific rules for small taxis, such as limited speed or determining expensive penalties. Taxi downsizing could also psychologically increase the popularity of using small cars due to the increasing usage in the taxi industry. The usage of small taxis will help promote the usage of Micro cars and Nano cars as taxis in the future, and even the usage of quadcopters for moving small taxi from one place to another place in the city. Even though we think in the first view taxi downsizing will find good popularity because of the lower fare. Also, taxi service companies can increase the number of small taxis in contrast with standard taxis in the city, based on the lower cost, which provides better accessibility for passengers and increases social satisfaction. Table 7
displays the benefits of small taxis on sustainable development indices.
Besides taxi downsizing, the government can encourage people to use small cars for developing sustainable transportation systems by providing car owners with subsidies for downsizing passenger cars. For example, in 2015, the local government offered a subsidy of 10,000 CNY for replacing old and high-polluting vehicles to electric cars in Xi’an, China [79
]. Those policies improved urban transportation systems to achieve a good step toward sustainability.
Taxis, as one of the urban public transportation modes, has considerably more mileage than private cars. Taxi downsizing has positive impacts on the environment, society, and economy. These three components are the primary concerns of city managers and urban planners for providing a sustainable city, where taxi downswing helps promote the sustainable development of urban transportation systems. This paper is aimed at finding a better fleet management mode for taxi vehicles. The usage of taxi trunks and the number of passengers for each taxi trip were investigated. According to the observations in Hangzhou, China, we found that 88% of taxi drivers did not use their trunk because passengers did not have any baggage and 60% of the taxis only picked up one passenger. Also, with the analysis of GPS data of 7081 taxis in Hangzhou, this paper found that the average speed of taxis was 23 km/h. The taxi downsizing policy was considered by replacing standard car taxis with small cars, and the vehicle mass can be reduced by 400 kg. Consequently, the annual reductions for a taxi vehicle in terms of fuel consumption, CO, HC, and NOx were estimated to be 1600 L, 311.9 kg, 15.4 kg, and 8.9 kg, respectively. Attention to safety based on vehicle weight reduction is essential after implementing the taxi downsizing. Due to the low average speed of taxis, the risk of fatalities in accidents can be significantly reduced. After decreasing the 400 kg weight of a standard taxi, the taxi downsizing has some influences on economic efficiency, e.g., 1760 USD (1600 L × 1.1 USD/L) saved for the annual fuel cost. Comparing a standard car and a small car, such as Hyundai Sonata and Peugeot 108, we found that replacing standard cars with small cars saves about 38% of the total cost.
The proposed policy of taxi downsizing could be expanded in the future research: (a) the management of electric taxis; (b) driver behavior in the acceptance of small vehicles as taxis; and (c) effects of small taxis on road capacity. This study provides the feasibility of taxi downsizing for urban and traffic managers. The future work will consider the right fare fee of a small taxi to evaluate all expenses and define a specific policy.