San Francisco is the 7th best city in the US for biking, according to Bicycling Magazine’s 2014 review. Recent additions of new bike lanes and other road infrastructure have helped the city become more friendly to bikers. According to SF Gate, an estimated 75,000 cyclists take to the streets each year — 9% of the SF population. 3.4% of SF workers commute by bike — far higher than the national average of 0.6%. That said, heavy bike and car traffic combined with distracted driving contribute to cycling danger in SF.
2014 SF Police data* shows that a bicycle/car collision resulting in injury or worse happens on average once a day. (This number is likely low, as not all bike collisions are reported.) We combined this data with historical collision records available from transbasesf.org to create a map of bike collisions over a 10-year period. Crash locations are mapped to their closest intersection in SF reporting, and in the past ten years, almost every intersection in the denser parts of the city has had at least one bike collision. Downtown, SoMa, and the Mission are the most collision-prone neighborhoods, with the commute paths down the Valencia Street and Market Street Corridors being the areas with the most dangerous intersections (recent improvements in bicycle infrastructure could well be changing that).
Since police data is only retrospective, we wanted to see how else we could use driving and cycling data to identify dangerous intersections and corridors. This new data source could give bicyclists a more comprehensive view of areas in the city that require more caution, or in some cases, complete avoidance.
Zendrive’s technology measures driving safety using only the sensors on a driver’s phone, analyzing the safety risk of a driver or fleet. Within the course of our work, we’ve tracked 1100 drivers over 37,500 miles across San Francisco, and measured cell phone use while driving, among many other safety metrics such as speeding or rapid acceleration. Any of these behaviors can be the cause of a collision with a bike, but distracted driving due to phone use while driving constitutes a special hazard to cyclists. We looked at intersections with at least one reported bike collision to see where drivers were on the phone while at high speed.
The areas with the most driving offenses are SoMa and the 19th Ave corridor — places where it is easy to speed when there’s no traffic, and places where the expectation of speed makes traffic jams more frustrating, and thus more prone to phone use. However, they also not always the places with the most bike traffic (19th Ave, for instance), so are not necessarily the most dangerous for cyclists. To find out what intersections are especially dangerous, we need to combine this dangerous driver index with cycling data.
We teamed up with Human to look at the areas of San Francisco with the most bike traffic — more bicyclists means a higher chance of a bicyclist being hit — then combined that data with our driver distraction map to discover where drivers and cyclists need to look twice before crossing.
Now that driver distraction is combined with cycling data, we can see where large numbers of cyclists mix with dangerously distracted drivers. There are many factors that influence the rate of collisions at a particular intersection. Some of the most important are simply how many cars and bikes pass through one. But for any individual cyclist, driver distraction poses particular danger that hasn’t yet been well studied. Here we can see how patterns of driver behavior can make certain corridors especially risky for cyclists, even if those intersections haven’t accrued as many crashes yet.
“This study underscores the need for a comprehensive, safety-first approach on our city streets,” said Tyler Frisbee, Policy Director at the San Francisco Bicycle Coalition. “San Francisco can reduce traffic deaths and serious injuries to zero, which is what the City has committed to with a concrete approach known as Vision Zero. Encouraging everyone to pay attention is a crucial part of improving our streets, and hopefully this study will re-emphasize the need to prioritize safety when designing our roadways.”
Of course, there are many factors that go into the safety of an intersection, with everything from protected bike lanes to weather and roadway wear and tear. But no matter where you are, if there’s a pattern of egregiously distracted driving in your route, it’s always good to take extra precautions when you bike!
*Police Data is available through a request from the SF Mayor’s Office.