There is a pressing need to estimate the magnitude and dynamics of the behavioural effects of transportation investments and policy. This article innovates by applying an experimental-control group research design to the case of new light rail transit service in Los Angeles, California. Only a handful of previous studies use an experimental design to assess impacts of light rail transit, and this is the first to use an experimental design to measure impacts on vehicle miles travelled, a key determinant of greenhouse gas emissions from the transport sector. We administered an annual seven-day travel study to a panel of households in the vicinity of Los Angeles’ Expo light rail line before the 2012 start of rail service and twice after the line opened. We find that households living within walking distance (1 km) of the new light rail drove approximately 10 fewer miles per day relative to control households farther away. Rail transit trips among near-station households approximately tripled relative to households beyond walking distance. Such driving reductions among households within walking distance of new rail transit stations suggest that Los Angeles’ large rail transit investment, coupled with land use policy, has the potential to help achieve climate change policy goals. More broadly, experimental evaluation can provide insights into causality and patterns of travel behaviour change associated with planning policies.