Since 2014, about 30 cities in the United States have adopted the goal of eliminating all traffic fatalities and severe injuries by 2025, known collectively as the Vision Zero pledge. Those cities agreed to undertake a range of changes—from improvements in street design to tightened traffic enforcement—in the name of boosting safety for road users.

So far, it’s been an uphill battle: Even as cars themselves become safer, we seem to be operating them more recklessly. Vehicle crash-related deaths have actually been increasing in urban areas since 2009, reversing a 30-year decline in road fatalities.

Those results suggest that it’s going to take more than piecemeal fixes to dramatically drive down traffic-related mortality. According to a new report from the American Public Transportation Association and the Vision Zero Network, there’s at least one a straightforward way cities can make a big difference: Improve local public transit. Places where people take more trips on public transit per capita have smaller proportion of road fatalities.

This analysis looks at the metro level for an early sense of how progress can be made towards Vision Zero. Using data from the National Highway Traffic and Safety Administration (NHTSA) and Federal Transit Administration (FTA), the study finds that large metro areas with higher levels of public transportation (more than 40 annual transit trips per capita) have lower traffic fatality rates than cities with fewer than 20 transit trips per capita.

“One of the most powerful traffic safety tools a city can employ to eliminate deaths and injuries due to road traffic crashes is its public transportation system,” Paul Skoutelas, president and CEO of APTA, said on a recent briefing call. “It takes just a modest increase in public transit use to result in a dramatic decrease in traffic fatalities.”

Two of the earliest Vision Zero cities with the largest per-capita transit trips stand out from the pack: New York City and San Francisco. Traffic fatalities have decreased significantly in the two cities in the last four years. Since 2014, traffic deaths are down 28 percent in New York City and 41 percent in San Francisco. (The historically low numbers in the two cities look good even when you only count the drop in pedestrian mortality, too—with a decrease by 46 percent and 34 percent respectively.)

On average, the 11 metro areas with over 40 transit trips per capita have relatively low traffic fatality rates, with 5.7 fatalities per 100,000 residents. Those 11 cities include Boston, D.C., Honolulu, Seattle, Philadelphia, and Chicago.

Overall, the 108 U.S. metro areas with more than 500,000 people average 9.9 fatalities per 100,000 residents. Size (and spawl) is a factor: As the chart below shows, the relationship between transit trips and fatalities is tighter for metros over 2 million people.

Metro areas such as Tampa, Orlando, Phoenix, Miami, St. Louis, Riverside, and Atlanta all have fatality rates above the APTA’s calculated average, correlated to their fewer transit trips. Just below that average is Houston, which holds the grim distinction of having the worst per-capita roadway fatalities among America’s 12 largest metro areas from 2001 to 2016, as the Houston Chronicle highlighted last week.

In previous research, APTA found that public transportation is ten times safer per mile than traveling by car in terms of traffic casualty rate. Rail is even safer—with 18 times fewer traffic casualties compared to driving. “At the NTSB, we investigate transportation disasters of all types, but by far, more Americans die on our roads than in any other mode,” Bella Dinh-Zarr, a board member at the National Transportation Safety Board said on the briefing call.

“So much of the cost-benefit consideration [of transportation safety] is done on a distance-based analysis, as opposed to a per-capita-based analysis,” said Vision Zero Network founder and director Leah Shahum. That vehicle-miles-traveled (VMT) formula helps in assessing car safety on highways, but doesn’t work as a metric for cities that are also trying to reduce driving in general. “Vision Zero is more than a catchy name or a slogan; it’s a fundamental shift in how we view safety,” she said.

While there’s a case to be made that bigger cities with dedicated public transit have other features that have reduced road fatalities (we’ll get to that in a second), there is a clear benefit to point to: With fewer cars to do dangerous things on the road, people are safer. Even just reducing the total VMT can pay off in reducing collisions: APTA estimates that with Americans taking over 1,300 trips per year, a shift of transit mode share from 1.5 percent to 3 percent could accomplish anywhere from a 10 percent to 40 percent reduction in traffic fatality rates.

“When I see public transportation data in isolation, it’s important, but it’s a signal in a sea of many signals that add up to the overall picture,” said Adie Tomer, a fellow at the Brookings Institution Metropolitan Policy Program. “Transportation is multifaceted; it’s multimodal. No place is simply going to use public transportation to get around and so safety conditions on our roads are an amalgamation of land use and mode choice.”

With pedestrian and bicycling deaths rising in recent years, the pursuit of Vision Zero has placed a new emphasis on those kinds of fatalities, but from a metro-level analysis, it’s still people in automobiles that make up the majority of road deaths and thus those have remained the focus of federal policy. That’s reflected in the many technological advances, from airbags to collision-detection software, that have helped make modern vehicles vastly less lethal to drivers and passengers. “The safety regime inside companies and inside government has been spent so much time designing our cars to be as safe as possible for the people inside them,” Tomer says.
But now we’re noticing what we’ve been missing outside the vehicle.”

One could make the argument that the strong connection between safety and transit in large cities makes cities like New York, San Francisco into outliers. But if the safest cities are the ones with the fewest drivers and fewest private cars, places like Tampa or Charlotte could see big safety improvements if they beefed up their public transportation options.

“Having a balanced set of investments, including mass transit, is going to be an important piece of the puzzle,” said Polly Trottenberg, director of New York City’s Department of Transportation. “There is a lot you’re hearing that cities are doing focused on roadway design, on enforcement, on education that any city can do to make their streets safer—to make them feel more acceptable to all users, and make them not only about cars.”