In 2003, the city of London took a dramatic step in the battle against traffic congestion: It implemented a congestion charge of £5 for those driving private vehicles into an eight-square-mile central congestion zone on weekdays between 7am and 6:30pm. The fees were increased twice, and since 2011 have stood at £10. Drivers purchase day passes online, and a camera network and a license-plate-recognition system allow for enforcement and penalty collection. Motorcycles, bicycles, taxis, and buses are exempt from the charges.

An essential aspect of London’s system is that it invests the revenue fees in public transportation — helping to make transit more attractive as driving downtown has become more expensive. And even without spending on more buses or drivers, the fee system helped. After the congestion charge began, bus congestion delays dropped 50%, while ridership increased 14%.

While the program has certainly succeeded in reducing traffic congestion, safety advocates have raised concerns about the possibility of faster traffic speeds resulting in more crashes. Now, the Washington Post’s Emily Badger reports on new research with an encouraging finding: Central London’s roads have actually become safer since the congestion charge began.

Colin Green, John Heywood, and Maria Navarro of the Lancaster University Management School analyzed crash data, and find a “substantial and significant” decline in serious and fatal crashes both within the congestion zone and in the surrounding areas following the system’s implementation. Badger summarizes the findings:

In total, the researchers conclude that congestion pricing in Central London has been associated with 30 fewer crashes a month in the area, a drop of about 40 percent. Meanwhile, they estimate that the policy has led to about 46 fewer serious and fatal collisions a year — and 4.6 fewer deaths.

This wasn’t an obvious result when the city began this experiment a decade ago. Buses, taxis, motorcycles and bikes are exempt from the charge, which now costs £10 a day (it’s enforced through a series of video cameras and license plate readers). It’s possible that any safety gains from having fewer cars on the road might have been wiped out by other changes created by the policy — if, for instance, faster travel turned fender-benders into major crashes, or if more taxis and bikes flooded the roads, or if cars simply changed when and where they drive to skirt the congestion fee.

A system that reduces both pollution and congestion benefits public health. If a city is going to expect a large portion of its workers and residents to switch from driving to riding the bus, though, it had better have a high-quality, affordable bus system. Congestion reductions can make for a faster bus trip, but that’s little help if the bus doesn’t stop near your home or workplace, or if it’s not sufficiently usable for parents with small children or people with disabilities. Congestion fees can fund expansions in transit service, but voters are likely to be skeptical of new service that won’t materialize until a congestion-charge system is implemented.

As far as making transit more financially attractive than driving, Seattle recently began discounting fares for lower-income residents — specifically, those whose household incomes are at or below 200% of the federal poverty level ($47,700 for a family of four in 2014). Rides are now $1.50 — a discount of more than 50% off peak fares — when riders use an ORCA smart card. King County transit officials anticipate getting ORCA cards to 100,000 qualified people, in part by relying on an outreach network developed to enroll people in health plans that recently became available under the Affordable Care Act. One driving force behind Seattle’s program is concern for the region’s economy, explains the New York Times’ Kirk Johnson:

The problem it addresses is that many commuters from places like SeaTac, an outlying suburb, are too poor to live in Seattle, where prices and rents are soaring in a technology-driven boom. If they are pushed out so far that they cannot afford to get to work or give up on doing so, backers of the project said, Seattle’s economy could choke.

“I would characterize this as a safety valve,” said Dow Constantine, the King County executive and chairman of Sound Transit, a transportation agency serving multiple counties in the region. From 1999 to 2012, Mr. Constantine said, 95 percent of the new households in King County have been either rich or poor, earning more than $125,000 a year or less than $33,000, with hardly anything in between.

It’s in everyone’s interest for people to be able to get where they need to go without spending hours sitting in traffic. Studying outcomes in cities like London and Seattle that try bold new approaches to transportation gives us important data to inform future transportation policies.