October 3, 2017

Gas anxiety and PHEV charging behavior

Combing an internal combustion engine and an electric powertrain, plug-in hybrid electric vehicles (PHEVs) offer the potential to reduce gasoline consumption while retaining the ability to travel long distances with fast and convenient refueling. They are inherently less dependent on recharging infrastructures than battery electric vehicles (BEVs), which can only be powered by electricity. However, data from the EV Project  showed a paradoxical finding: PHEV drivers, for whom recharging is optional, tend to plug in more frequently than BEV drivers, for whom recharging is necessary. Several press articles took this as a sign that PHEV drivers have “gas anxiety” – a play on the common term “range anxiety” – due to their apparent desire to avoid using gasoline. Working with Professor David Keith from MIT, Professor MacKenzie and I have just published a new paper testing for evidence of gas anxiety empirically. (As usual, a non-paywalled manuscript is available on our publications page.)

We gathered data using a web-based stated preference survey of PHEV drivers to determine how situational variables such as charging power, the respective prices of gasoline and recharging, and the vehicle’s state of charge influence PHEV owners’ charging decisions. This means we presented PHEV owners with hypothetical situations, and asked if they would charge their car in such a situation. The distribution of the respondents is shown in the map below. We used the respondents’ answers to fit a model of charging choices, from which we estimated the relative weights that PHEV drivers place on the cost of gasoline and cost of electricity on a distance–equivalent basis. We used a type of model called a latent class model, which allows us to identify different modes of behavior common to different groups of respondents.

We found that later PHEV adopters and those with primarily financial motivations valued expenditures on gasoline and electricity equally. We call it this the cost-minimizing group, because their charging choices are what we would expect from a group trying to minimize overall expenditures.  Earlier adopters with other motivations such as environmental concern and vehicle performance were willing to pay about four times as much for electricity as for the gasoline that same electricity will displace. We call this group the gas anxiety group because their behavioral pattern  – being willing to pay a premium to replace gasoline with electricity – is consistent with the concept of gas anxiety.

According to our sample, the gas anxiety group is currently the majority: 66%. But it is hard to say whether future PHEV owners will split along these lines since the variables predicting class membership (years of PHEV ownership and prevalence of financial motivations) are likely to change over time.  What seems to matter here is why the length of PHEV ownership has this influence on the class membership. One possibility is that as a person owns a PHEV for a longer time, their preferences for charging evolve because of the increased familiarity with the charging system or the acquired taste for the driving characteristics of electric propulsion such as low noise and instant torque. If this is true, the proportion of gas anxiety group is likely to grow with the increase of the number of the PHEV owners with longer ownership. Another possibility is that the earlier adopters are inherently different from the later adopters: they might be more enthusiastic about technology and more pro-environmental than the general population. If this is the case, with more new adopters joining the market, the proportion of gas anxiety group will gradually decrease.