The data collected by the fitness app Strava turns out to be a pretty accurate way to get a handle on how many people commute on foot or by bike, say scientists with the Centers for Disease Control and Prevention. This could help urban planners as they try to make cities safer for walkers and bikers who commute to work.

Cities trying to become more bike- and pedestrian-friendly need to know how many people are active commuters. Commuting, however, is really hard to observe, so typically to collect that information, groups like the US census rely on surveys. But surveys are notoriously unreliable, because people are forgetful or because they lie. In fact, people are so bad at reporting what they eat that scientists argue food diaries are screwing up studies into obesity.

Fitness apps like Strava — which bills itself as a social network for athletes — collect data about how people move around using GPS, which is less subjective. Some cities are already using its data aggregation and analysis spinoff, Strava Metro, for city planning. But fitness apps have their own problems — since the people who use them probably aren’t all that representative of the broader population. For one thing, people who use fitness apps probably value physical activity. They also probably have the money and the time to spend on leisure activities.

So to double-check Strava’s tracking data, scientists with the CDC compared it with census data in four US cities: Austin, Denver, Nashville, and San Francisco. And it turns out that the Strava data tracked pretty closely with what the surveys reported. The number of walkers and bikers counted by Strava and estimated by the surveys seemed to align most closely in especially dense cities like San Francisco.

This trend in urban planning seems to be catching on, with a reported 70 cities using Strava’s data, according to Strava Metro’s website. Other organizations like the Alliance for Biking and Walking are also trying to figure out how best to track people who commute without buying something easy to count, like a metrocard, a parking pass, or a car registration.

The CDC is encouraging cities to think about how urban environments shape people’s health, as part of its Healthy Community Design Initiative. The CDC hopes that as cities become safer for walking and biking, people will use their commute as a time to be physically active. And safer, more bike- and pedestrian-friendly cities would mean fewer car accident fatalities, and less lung-irritating pollution. Plus, in many places, walking and biking are cheaper and faster options. Being able to get more accurate, timely information about commuting habits is a necessary first step toward that goal.