Time scarcity is one of the strongest correlates of fast food consumption. To estimate the causal effect of time lost on food choice, we match daily store-specific foot traffic data traced via smartphones to plausibly exogenous shocks in highway traffic congestion in Los Angeles County. We find that on days when highways are more congested, individuals are more likely to frequent fast food restaurants and less likely to grocery shop. In our main model, a one standard deviation increase in traffic delay leads to a 1% increase in fast food visits, equivalent to 1.2 million more fast food visits in Los Angeles County per year. The effects are particularly pronounced for afternoon rush hour traffic. Our results imply a net reduction in healthy food store choice due to time lost.