SFMTA Left Turn Traffic Calming Project
In 2020 and 2021, QC led the data collection effort for SFMTA's Left Turn Traffic Calming project. The project aimed to reduce unsafe left turns at several intersections in the city.
In 2019, 40% of traffic deaths in San Francisco were caused when drivers made left turns and didn’t see the person in the crosswalk, until it was too late. By adding traffic-calming treatments to intersections that encourage drivers to take left turns more slowly (5 mph is ideal) and at a 90-degree angle—as opposed to speeding through at an angle—drivers have more time to yield to people in the crosswalk.
To combat this deadly trend, the city installed waist-high vertical delineator posts, small rubber speed bumps, and paint to create enhanced center lane lines and painted safety zones to encourage slower, wider left turns and increase drivers’ awareness of other road users.
To measure the impact of these installations, Quality Counts collected left turn driver speeds before and after the installation at the target intersections, as well as additional control intersections throughout the city. QC field technicians used lidar guns and positioned themselves to capture the vehicles turning speed as it turned left and entered the crosswalk where conflict with a pedestrian may occur.
QC also used cameras to concurrently capture turning movements at these intersections along with pedestrian/vehicle yielding and close call data. This data was vital in measuring the success of the installations.
Overall, SFMTA found a 17% reduction in average speed (1.7mph slower) and a 71% reduction in the likelihood of a car turning left at higher speeds over 15mph at seven high-crash test intersections. The result of this project showed that these treatments are proven to slow driver speeds and encourage safer left turns for both people in the crosswalks and opposing traffic.