A month-long field experiment evaluated the impact of descriptive social norm information on self-reported reduction of private vehicle use. Following a baseline week, participants were asked to reduce their vehicle use by 25% and were randomly assigned to a control condition or to a low or high social norm condition in which they received information that either under- or over-reported others’ successful efforts to switch to sustainable transportation. Results indicated a significant linear trend, such that messages highlighting more prevalent descriptive social norms increased sustainable transportation behavior (relative to private vehicle use) for commuting, but not non-commuting, purposes. Participants in the high social norm condition decreased their commuting-related private vehicle use by approximately five times, compared with baseline. Car-use message campaigns can reduce private vehicle use by highlighting descriptive norms about others’ sustainable transportation efforts, but these messages appear to be most effective for commuting behavior.