Experimental Design
The experiment consists of two interventions run in series: an informational treatment run over 19 days from October 5–23, followed by two phases of financial treatment run over 27 days from October 24 to November 19. Interventions were conducted within one academic quarter to maintain consistency in campus population and schedules.
In the informational intervention, half of the study participants were randomly assigned to treatment and half to control. Treatment consists of an email, delivered three times (once per week), stating the climate benefits of daytime charging compared to nighttime charging. In each email, benefits are reported as avoided CO2 emissions, equivalent unburned gasoline, and prevented global environmental damages.
In the financial intervention, drivers were given discounts for all Level-2 charging and randomly placed into treatment arms that varied discount size. The financial intervention consists of two phases. During the first phase (October 24 to November 5; 13 days), roughly one-third of participants receive a small discount ($.16/kWh), and two-thirds receive a large discount ($.23/kWh) - equivalent to 50% and 75% off the base campus rate of $.30/kWh, respectively. We set discounts so that the effective small-discount rate of $.14/kWh corresponds to the cheapest overnight home charging rate of the local electric utility, thus negating any economic advantage of overnight home charging. The large discount rate of $.07/kWh is equivalent to the mean LMP of wholesale electricity at UCSD, corresponding to the plausible lowest cost drivers would pay for charging.
During the second phase (November 6–19; 14 days), half of the large discount group continues with the large discount, while the other half moves to the small discount. To ensure parity in active campus days across both financial treatments, we schedule the second treatment to span 14 days, accounting for Veterans Day on November 10, when commuters are likely absent from campus. The second financial intervention thus has three treatment arms—LL (Large-Large), LS (Large-Small), and SS (Small-Small) discounts—given to three distinct groups. In this phase, we test for the presence of habit formation when financial discounts are reduced. If the charging behavior of participants on reduced discounts (LS) closely mirrors those who continue to receive the large discount (LL), our results are consistent with habit formation. In contrast, if the charging behavior of participants on reduced discounts (LS) reverts to those receiving the small-small sequence of discounts (SS), our results indicate the absence of habit formation during the first discount.
Four months after our series of informational and financial experiments, we ran a follow-up intervention over 13 days from February 5 to 17 to test for incentive-induced perceptions of scarcity of available chargers. In this intervention, we varied the discount notifications such that the messages to treated and control groups implied that different numbers of drivers would receive the discount. This follow-up experiment mimicked phase 1 of the financial experiment: the same methodology, same participants (i.e., new club enrollees were excluded), stratified block randomization into treatment arms that receive small or large discounts, notifications, and odometer surveys. In total, the experiment consisted of four treatment arms. Two arms received the large discount; two received the small. New to this experiment was that, within each discount regime, half of participants received a discount notification email that indicated that all drivers would receive the discount simultaneously, while the other half received a discount notification message that indicated that no more than 33% of drivers would receive the discount