Social distancing reduces the transmission of COVID-19 and other airborne diseases. To test different ways to increase social distancing, we conducted a field experiment at a major U.S. airport using a system that presented color-coded visual indicators on crowdedness. We complemented those visual indicators with nudges commonly used to increase COVID-19 preventive behaviors. Analysing data from 57,146 travellers, we find that visual indicators and nudges significantly affected social distancing. Introducing visual indicators increased the share of travellers practicing social distancing, and this positive effect was enhanced by introducing nudges focused on personal benefits (“protect yourself”) and public benefits (“protect others”). Conversely, an authoritative nudge (“don’t break CDC COVID-19 guidelines”) did not change social distancing behavior. Our results demonstrate that visual indicators and informed nudges can boost social distancing and potentially curb the spread of contagious diseases.