Marin County to Install Bicycle Sensors at 31 Intersections

green_light.jpgFlickr photo: crook inc

Cyclists know all too well the frustration of traffic lights that only change when cars activate a ground sensor, but are not tripped when bicycles arrive. Now Marin County is about to give bike riders a green light. The county will install sensors at 31 intersections (PDF) in 10 cities so that a cyclist’s arrival activates a traffic light change.

Traditional traffic signals come in two versions: fixed time lights for which everyone waits and demand-activated signals that use sensors that recognize when a car is waiting, usually through an electromagnetic sensor embedded in the pavement. As most city-dwelling bike riders know, if you hit a demand-activated light you either wait until a car arrives, press the pedestrian "walk" button to activate the signal or blow through the intersection on red. It’s a safety issue for the county and groups that pushed the change, including the Marin County Bicycle Coalition.

The bike-friendly signal project will use three different types of sensors to spot waiting bicyclists, according to Pat Echols, Marin Public Works Senior Civil Engineer. Depending on the space and mechanics of a given intersection they will use an electromagnetic pavement sensor, similar to the ones for cars, or a wireless pavement sensor that uses microwaves to detect the bikes or lastly a real-time video camera that tells the traffic signal’s computer that it sees a bicyclist waiting, according to Echols.

The video cameras do not take photos and are not used for traffic enforcement, but solely for spotting bikes or cars to activate a signal change, he said. These demand-to-activate sensors as they are known, may begin going in as early as this fall if all the approvals can be wrangled from each city and the state, according to Echols. But it is a tight window of time and the installation work may not start until next spring. But they are coming. Once underway the project should take two months to complete.

The County set aside $922,000 for the project, out of a $25 million federal pilot project to promote bicycling and walking in Marin County. Currently $20 million in projects (PDF) have been identified.

The lights include four intersections in Corte Madera, two in Fairfax, two in Larkspur, one in Marin City, two in Marinwood, three in Mill Valley, 10 in Novato, three in San Anselmo, two in San Rafael and two in Sausalito.


The red areas mark close calls between drivers and pedestrians. Image via City of Bellevue

Can Algorithms Design Safer Intersections?

Cities and tech firms are deploying new technology to gauge risks at dangerous intersections. These sensors, cameras, and machine-learning algorithms are promising, especially when it comes to measuring close calls that don't result in crashes - but cities are still figuring out how they can use this information. In the meantime, there's no reason to wait on designing safe streets.