eVMT Project

The growth of the plug-in electric vehicle (PEV) market carries with it great potential environmental benefits. However, there is much uncertainty about how consumers actually use PEVs, and because of this it also remains unclear how to quantify benefits such as reductions in greenhouse gas emissions. The purpose of this project is to monitor how PEVs are used day-to-day and to gain an understanding of the true environmental benefits yielded by their expansion. This study is the most in-depth look at PEV use so far, and the detailed understanding of the use patterns of PEVs will help policymakers determine the most effective strategies and policies for encouraging electric miles, and what those electric miles can mean in terms of environmental effects.

This project is ongoing and is projected to be completed in Summer 2019.


Executive Summary

The Advanced Plug-in Electric Vehicle (PEV) Travel and Charging Behavior project (project) provides a platform to monitor how new plug in electric vehicles (PEVs) are being used on a day to day and month to month basis within the household travel context by placing data loggers in participant households for a period of one year. This provides two advantages over simply studying the PEV for a long period or studying the household for a short period. First, only studying the PEV does not give a clear indication of what role the PEV is playing in the household and what travel needs are not covered by the PEV. Second, the shorter-term studies do not capture those infrequent events such as long trips which may have a bearing on the purchase or lease of the vehicle. This project represents the first step in understanding these important dynamics and potential barriers that need to be addressed in the transition to cleaner vehicles.

The project consists of a large survey of PEV owners/lessees, followed by intensive study of a subset of those respondents. Data monitors that collect GPS, state of charge, speed, revolutions per minute (RPM), charging events, and numerous other parameters on a nearly second-by-second basis were placed in all the vehicles in the selected households for a period of one year. The project is about midway through data collection; the first phase of the study was completed in late summer 2016, which included logging all the vehicles within 72 PEV households for a full year. Data is currently being collected from an additional 132 households, and the final number will be 264. These respondents own or lease one of the following seven PEVs with their EPA electric ranges: Toyota Plug-in Prius (PHEV11), Ford C-Max Energi (PHEV20), Ford Fusion Energi (PHEV20), Chevrolet Volt (PHEV36,53), Nissan Leaf (BEV73,86,105), BMW i3 REx (PHEV73), and Telsa Model S (BEV200+). The BMW i3 REx and Tesla are not included in many of the results presented as they were not part of the first 72 households in the first phase of this study. The results presented in this interim report comprise a combination of survey responses and results from the year-long data collection.

Results so far suggest that participants maximize the use of their PEVs, which account for more than half of household miles except in the case of the Leaf. The Leaf is used intensively as well, but the 18 Leaf households in this study had more licensed drivers than households with another PEV type, making direct comparisons difficult. Leafs were less likely to be used on days that required charging to complete a day’s travel, whereas PHEVs were more likely to be used as the distance of travel increased, and in fact PHEVs were likely to be chosen more frequently than ICEs for long travel days. Leafs were used less than 50% of the time if the day’s travel required one charging event, and very rarely if it required two charging events. In contrast, PHEVs were used more than 50% of the time for travel days that would have required the Leaf recharge at least once, but dependence on gasoline for those days was very high. These results suggest that as BEV range increases, trips done in ICEs may shift to the BEV in order to maximize travel in that vehicle, as long as it fits into the range needed for that day.

Charging and usage of PHEVs differs by electric range such that the usage of a PHEV36 is much different than a PHEV11. Instead of PHEVs with smaller range plugging in more, they plugged in less overall during the year-long collection period. We analyzed survey data to investigate the factors that influence the decision to plug in at the workplace, such as price, congestion, time limits on parking, and external factors such as income. Our analysis shows that the likelihood of plugging in is linked to the electric driving range that can be recovered per charging event. PHEV11’s “motivation” was limited to 11 miles of motivation, whereas PHEVs with larger battery sizes showed increased motivation to plug in up to the point that range recovered equaled their vehicle range. This may also help explain why the likelihood of abandoning plugging in increases as electric range decreases. Also, fewer people plugged in in 2016, when gasoline prices were approximately 20% lower compared to 2015. Increasing PHEV range has benefits in both the technical ability for a vehicle to do more travel with electricity and by increasing the motivation to plug in.

Criteria emissions are also important when considering the overall environmental benefits of PHEVs. All PHEVs have an engine and therefore emit tailpipe criteria pollutants, which are related to the frequency and conditions at the time of engine starts. If the engine is able to warm up before providing power, emissions are lower than starts when the engine must immediately provide power (which is likely to occur most frequently in blended PHEVs with low range). This analysis shows that at least 17% of PHEV11 engine starts were high-power cold starts. If those starts are significantly more polluting, blended PHEVs may not provide criteria emissions benefits compared to conventional vehicles.

Finally, on a GHG basis under the current electric grid emissions, low range PHEV households show lower household GHG emissions per mile ([all ICE+PEV miles]/[all associated GHG]) of travel than in those households with longer electric range vehicles. Because their gasoline efficiency is high and travel is shifted to that vehicle, this displaces other less efficient gasoline miles. However, under a zero carbon electricity scenario, GHG per mile of household travel is more favorable for longer range PEV households.

PEVs in this study were being used intensively and were preferred over the ICEs for household travel. BEVs were used more than their ICE counterparts within the limits of their range, but were less likely to be used if they require charging during the day. Increasing BEV range may result in BEVs being selected more often for journeys that exceed the range of current BEVs. The question remains, however, whether increasing range will simply shift travel within the household on longer travel days, or if it will result in a BEV becoming a viable option for ICE replacement in more households. PHEVs are currently very capable ICE replacements, but analysis thus far shows that increasing range increases the likelihood that PHEV travel will take advantage of these vehicles’ electric capabilities. Finally, blended PHEVs may have some drawbacks in terms of emissions benefits pending further analysis of the effect of high-powered cold starts. Providing more motive power through the electric motor increases the benefits that PHEVs provide.

Read the full interim report here.


Selected Figures