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Electric vehicles (EVs) are energy efficient and often presented as a zero-emission transport mode to achieve long-term decarbonization visions in the transport sector. The implementation of a sustainable transportation environment through EV utilization, however, requires the addressing of certain cost and environmental concerns such as limited driving range and battery-charging issues before its full potential can be realized. Nevertheless, a specific type of use of EVs, namely in taxi services, may elicit positive public opinion, as it promises a commitment toward sustainability in urban life. In light of this, this study proposes an integrated approach that combines EV operation with a conceptual design for shared-ride taxi services. As some productivity loss may be naturally expected due to the time spent in charging, it is important to look at whether such performance loss from the passenger and system standpoints can be offset with ingenuity in operational design. In this study, an EV taxi charge-replenishing scheme that can be coupled with a real-time taxi-dispatch algorithm is designed. Sustainable Urban Mobility: the proposed EV charging schemes for taxi services are studied via simulations and the effects of the limited driving range and battery-charging details are examined from a system performance viewpoint. The simulation study also reveals illustrative results on the impact of the EV taxi fleet’s operation on the charging system. Next, a real-time shared-taxi operation scheme that allows ride sharing with other passengers is proposed to maximize the operational efficiency. The simulation results suggest that the shared-taxi concept can be a viable option to improve on the limitations caused by EV operation. In addition, the importance of projected charging demands and queue delays at different charging locations are also addressed. Some limitations and a future research agenda are also discussed.

1. Introduction – Sustainable Urban Mobility

The transport sector is responsible for about a third of all greenhouse gas (GHG) emissions in the USA and a fifth of worldwide GHG emissions (Buehler & Pucher, 2011 Buehler, R., & Pucher, J. (2011). Sustainable transport in Freiburg: Lessons from Germany’s environmental capital. International Journal of Sustainable Transportation, 5, 4370. Retrieved from[Taylor & Francis Online], [Web of Science ®][Google Scholar]; Ogden & Anderson, 2011 Ogden, J., & Anderson, L. (2011). Sustainable transportation energy pathways: A research summary for decision makers. Davis, CA: Regents of the University of California. [Google Scholar]). Naturally, reducing carbon-based vehicle emissions and GHG has been a critical issue because of their serious impact on the environment and human health in many urban areas. Two major approaches to address this have emerged in sustainable transportation research: (1) developing energy-efficient technologies and (2) adopting efficient transportation operations. As part of the energy strategy, auto manufacturers are producing alternative-fuel vehicles (AFV) such as hybrid vehicles (HV), plug-in hybrid electric vehicles (PHEV), electric vehicles (EV), and fuel-cell vehicles (FCV) that have zero or significantly reduced vehicle emissions. There is growing interest in EVs because they are considered to be zero-emission vehicles (ZEV) as they use no internal combustion engines (ICE) and are powered by electric motors from battery packs without any local CO and NOx emissions. Nevertheless, other concerns limit EV utilization. These include higher prices of vehicles and cost of the charging infrastructure, shorter driving ranges compared to the range of gasoline vehicles, and the lost productivity when vehicles are idle while battery charging. Even if EVs are not yet considered to be ready to replace ICE vehicles on a large scale, they can certainly be considered as a viable option for many fleet applications that envisage the use of energy-efficient vehicles and require fewer resources to operate than even personal automobiles (Barth & Todd, 2001 Barth, M., & Todd, M. (2001). User behavior evaluation of an intelligent shared electric vehicle system. In Transportation Research Record: Journal of the Transportation Research Board, 1760, 145152.[CrossRef][Google Scholar]; Green Car Reports, 2010 Green Car Reports. (2010). Better Place launches battery-swap test in four Tokyo taxis. Retrieved from [Google Scholar]; Blosseville, Massot, & Mangeas, 2000 Blosseville, J. M., Massot, M. H., & Mangeas, M. (2000). Technical and economical appraisal of Paraxitéle trial. Paper presented at the Seventh World Congress on Intelligent Transportation Systems, Turin, Italy. Retrieved from https://
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As examples of cities focusing on EV use as a sustainable option, the City of New York has set a goal of having one third of the city’s taxis electric by 2020 (Flegenheimer, 2013 Flegenheimer, M. (2013, April 21). Electic-taxi experiment asks if city’s cabbies can spare the time to plug in. The New York Times. Retrieved from [Google Scholar]) and the Seoul Metropolitan Government in Korea has unveiled its Electric Vehicle Delivery Plan for Seoul, which has set 72,000 EV and hybrid taxicabs as the green car provision target in the near future (Energy Smart Communities Initiative, 2011 Energy Smart Communities Initiative. (2011). Electric vehicle delivery plan for Seoul. Retrieved from [Google Scholar]). It is true that such electric taxi programs will be a great opportunity to improve sustainable air quality and energy consumption. However, the deployment of commercial EV fleets can raise concerns related to the complexity of the taxi industry and charging infrastructures. According to a recent study on the EV taxi pilot program by the Taxi and Limousine Commission in New York City (2013 Flegenheimer, M. (2013, April 21). Electic-taxi experiment asks if city’s cabbies can spare the time to plug in. The New York Times. Retrieved from [Google Scholar]), taxi drivers will require 45 to 90 min of charging during their daily service time, which is a critical loss in such commercial vehicle operations. Not being able to operate during charging breaks causes obvious revenue loss. Moreover, the shortage of vehicles at peak service times can decrease service quality because of the lower service acceptance rate. In the New York City pilot program, drivers are given certain privileges to turn down passengers based on their destinations to relieve concerns about the battery losing charge in the midst of their trips, and this causes a further loss of revenue. Clearly, the success of EV taxis requires efficient operations as well as fuel-efficient vehicles. Alternatives such as the e-hailing system suggested by the study (New York City, 2013) can help increase system efficiency through a reduction of cruising time, but such options might not be addressing the operations from a fundamental perspective. These potential issues motivate the authors to study potential EV taxi operation with a ride-sharing scheme.
As the traditional transportation system designs may render these limitations of EVs prohibitive, it becomes important to consider if newer paradigms of shared-ride operation can help offset it. The reasons to consider taxis with such shared-ride operations are as follows. First, taxi is certainly the most popular on-demand dial-a-ride system in many urban areas of the world. Second, typically short taxi trips within smaller areas are particularly suitable when EV driving range is an issue. Finally, due to substantial driving without passengers and fewer average number of riders than even private vehicles, taxis are more polluting than private autos (on a per-trip-distance basis), and thus replacing them with EVs is attractive. It is indeed easier, however, to improve this using better ride sharing, unlike with private auto use. Despite the cost of initial investment, utilizing EVs for taxi may prove to be an ideal application in terms of reducing emissions and fuel costs as shown in the 1-year EV taxi pilot program in China (New York Daily News, 2011).
In general, employing real-time ride sharing can be a great option to maximize the conventional taxi and paratransit operations. Such a concept dynamically utilizes the empty seats by assigning passengers on demand, which is quite different from the early version of carpooling projects that were not feasible for real-time response due to the lack of advanced information technologies. For instance, newer designs of sustainable demand-responsive transit (DRT) have been introduced in recent years because the provision of traditional public transport services for medium or lower demands has always been criticized for its operational inefficiency (Dial, 1995 Dial, R. B. (1995). Autonomous dial-a-ride transit introductory overview. Transportation Research, 3C (5), 261275.[CrossRef][Google Scholar]; Cortés & Jayakrishnan, 2002 Cortés, C. E., & Jayakrishnan, R. (2002). Design and operational concepts of high-coverage point-to-point transit system. In Transportation Research Record: Journal of the Transportation Research Board, 1783, 178187.[CrossRef][Google Scholar]; Quadrifoglio, 2007). At the same time there are also commercial enterprises proving that newer concepts for smaller vehicles, such as private cars and taxicabs, can indeed be introduced. For instance, Zimride and Carma are services recently initiated in the United States in the private sector, which facilitate ride sharing in private cars by simply matching drivers and riders in real time for passenger travel in urban areas. These services utilize vehicles operated by regular car owners and not commercial drivers. Similarly, real-time shared-taxi operations can also potentially show efficiency improvements.
Shared-taxi service can be characterized as an on-demand ride-share service operated by an online dispatch center such that the system is capable of taking service requests from individual customers in real time and establishing service vehicle schedules. Even though taxi sharing is prohibited by law in many cities in the United States, these services are being initiated in many countries. In China, the Beijing government recently allowed taxi sharing because of the shortage of taxicabs during rush hours. In Taiwan and Japan, shared-taxi services are conducted or have been initiated to link passengers dynamically (Tao, 2007 Tao, C. C. (2007). Dynamic taxi-sharing service using intelligent transportation system technologies. In Proceedings International Conference on Wireless Communication, Networking and Mobile Computing. pp. 3209–3212, Shanghai, China:IEEE. [Google Scholar]; Tsukada & Takada, 2005 Tsukada, N., & Takada, K. (2005). Possibilities of the large-taxi dial-a-ride transit system utilizing GPS-AVM. Journal of Eastern Asia Society for Transportation Studies, 22, 110. [Google Scholar]).
Recently, there have been studies on EV deployment with regard to environmental impacts, effects of driver behavior, site selection for charging locations, and smart grid solutions (Khan & Kar, 2009; Smart & Schey, 2012 Smart, J., & Schey, S. (2012). Battery electric vehicle driving and charging behavior observed early in the EV project. SAE International Journal of Alternative Powertrains, 1, 2733.[CrossRef][Google Scholar]; Ghamami, Nie, & Zockaie, 2016 Ghamami, M., Nie, Y., & Zockaie, A. (2016). Planning charging infrastructure for plug-in electric vehicles in city centers. International Journal of Sustainable Transportation, 10, 343353.[Taylor & Francis Online], [Web of Science ®][Google Scholar]; Giménez-Gaydou, Ribeiro, Gutiérrez, & António, 2016 Giménez-Gaydou, D. A., Ribeiro, A. S. N., Gutiérrez, J., & António, A. P. (2016). Optimal location of battery electric vehicle charging stations in urban areas: A new approach. International Journal of Sustainable Transportation, 10, 393405.[Taylor & Francis Online], [Web of Science ®][Google Scholar]). Khan and Kar (2009) reviewed HV technologies and examined various factors affecting consumer acceptance of HVs and EVs. Smart and Schey (2012 Smart, J., & Schey, S. (2012). Battery electric vehicle driving and charging behavior observed early in the EV project. SAE International Journal of Alternative Powertrains, 1, 2733.[CrossRef][Google Scholar]) conducted a survey study that collected data on the charging behavior of 2,903 private EV owners (e.g., Nissan LEAF and Chevrolet Volt) in the United States. They analyzed the EV charging patterns and locations and showed that the state of charge (SOC) of the battery at the start of charging events of private EV drivers was mostly distributed between 20% and 80%. Li and Zhang (2012 Li, G., & Zhang, X.-P. (2012). Modeling of plug-in hybrid electric vehicle charging demand in probabilistic power flow calculations. IEEE Transactions on Smart Grid, 3, 492499.[CrossRef], [Web of Science ®]
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) proposed a methodology of modeling the overall charging demand of PHEVs for a local residential community. The study assumed various key factors that determine the charging behavior and carried out Monte Carlo simulations. The location selection model for EV charging stations by Giménez-Gaydou et al. (2015) used a gradual maximal covering model based on daily charging needs, charging coverage, and adoption potential. They used socioeconomic characteristics and drivers’ activity patterns for charging demand estimation. Ghamami et al. (2016 Ghamami, M., Nie, Y., & Zockaie, A. (2016). Planning charging infrastructure for plug-in electric vehicles in city centers. International Journal of Sustainable Transportation, 10, 343353.[Taylor & Francis Online], [Web of Science ®][Google Scholar]) considered existing parking lots in city centers as candidate locations for PHEV charging by assuming that the charging demands can be estimated by office buildings near the city parking lots.
Commercial EV fleet operations have a set of unique characteristics that are not sufficiently addressed by the studies in the literature. For instance, the cost of recharging time for fleet vehicles is quite different from what is incurred by private drivers who can recharge when they return home for the night. Additional costs are involved to set up the charging infrastructure, such as EV charging locations for the fleet and allocating optimal number of chargers. The main factors in determining how often and where vehicles will need to charge cannot be simply estimated on the basis of vehicle mile traveled (VMT) by the EV fleet. Instead, exploring EV charging profiles in spatial and temporal scopes will be required to find out optimal strategies for EV fleet operations and allocating charging resources. There has not been much focus on how the EV fleet characteristics influence the charging behavior, thereby impacting the operational performance of the fleet and the charging infrastructure. Furthermore, there has been no consideration of flexible transportation solutions that may be particularly applicable for larger use of EVs than the limited fleet contexts discussed so far.
Investigating EV charging patterns in conjunction with shared-ride systems requires simulation modeling and analysis due to its complexity and nonlinear system performance. A recent simulation study conducted by Jung and Jayakrishnan (2012 Jung, J., & Jayakrishnan, R. (2012). High-coverage point-to-point transit: Electric vehicle operations. In Transportation Research Record: Journal of the Transportation Research Board, 2287, 4453.[CrossRef][Google Scholar]) is an example of larger scale EV fleet application for DRT. The study assumed an innovative transportation alternative (Cortés & Jayakrishnan, 2002 Cortés, C. E., & Jayakrishnan, R. (2002). Design and operational concepts of high-coverage point-to-point transit system. In Transportation Research Record: Journal of the Transportation Research Board, 1783, 178187.[CrossRef][Google Scholar]) called high coverage point-to-point transit (HCPPT), which involves a sufficient number of deployed small vehicles for point-to-point passenger travel services with associated hub terminals for passenger transfers. However, this study mainly focused on the HCPPT-style design for EV operation rather than on investigating the general impact of vehicle charging. In this study, we describe an approach to simulate EV fleet charging profiles that employ an EV taxi-charging scheme and investigate the impact of the scheme on the system-level performance of taxi services. The main objectives of this article are to model a case of a large fleet of vehicles providing shared rides in a real-time demand-responsive manner and to discuss the specific issues that may affect the feasibility of the implementation of EV fleets for such taxi services. This article makes the following contributions:

  1. We develop a detailed simulation framework that can efficiently incorporate an EV-charging scheme with a real-time taxi-dispatch algorithm. The charging scheme includes the aspects necessary to model the expected inefficiency of EVs and identify charging demand characteristics.

  2. A series of simulation runs is performed with a fleet of 600 EV taxicabs on a network in Seoul, South Korea, that spans 605 km2 (233 mi2) and has 22 candidate refueling locations. The simulations focus on investigating the effects of the limited driving ranges, battery replenishing times, and initial states of battery on passenger delivery and peak charging demand.

  3. We suggest a shared-taxi strategy that improves the productivity loss that can be caused by an EV charging break. By introducing various levels of shared-taxi operation parameters, it achieves better performance in passenger delivery despite the critical limitations of EV fleets such as short driving ranges and regular charging breaks.

  4. Finally, our simulation study examines vehicle queue delays when the number of chargers is limited at each location. The simulation results provide specific insights into queue lengths, which can be the foundation for an optimal charger allocation model.

In the next section, we propose an EV-charging scheme for taxi service. Then, detailed simulation assumptions and scenarios are introduced in the subsequent section. Finally, we discuss the simulation results in terms of recharging demands, system performance, quality of service, and queue delay.
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