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How Intersections Now Sense What Will Zoom By

In Pleasanton, Calif., radar-like sensors can more easily detect bicycle traffic, providing safer crossings for cyclists and motorists.

On any given day, cyclists could end up in dangerous collisions. Sure, they can take precautions like wearing reflective clothing and cycling defensively, but even these may not fend off an accident with a motorist. While only 11 percent of bicycle accidents involve a car, 45 percent of these accidents take place at traffic intersections, according to legal website Nolo. But in Pleasanton, Calif., cyclists can ride through intersections a little safer knowing that technology is on their side.

Pleasanton is recognized as the first in the nation to test out microwave motion and presence sensors called the "Intersector," according to the Contra Costa Times. Much like how a police officer's radar gun works, the Intersector sends out microwave pulses and measures the reflection off a cyclist or motorist from up to 300 feet away.

When the sensor detects a bicycle, it triggers the timing of traffic lights one way, and when it detects only cars, it triggers a different timing scheme. The better pacing of green lights provides a safer crossing, says Pleasanton Senior Transportation Engineer Joshua Pack. The city tested its first sensor in January 2010; six additional intersections should have the sensors by the end of November, Pack says.

When it comes to bike safety, Pleasanton is neither the safest Bay Area city nor the most dangerous. But Alameda County (where Pleasanton is located) had the second highest number of bicycle accidents in Northern California from 2005 to 2009, according to The Bay Citizen. Over 3,400 cyclists in Alameda County were involved in collisions -- approximately 1,150 more collisions than San Francisco had during the same time period.

Pleasanton has sought out ways to provide safe cycling for many years, Pack says, noting the double standard that exists for bicycles. They're considered vehicles, but often must abide by pedestrian signals. "They're allowed on the roadway," Pack says, "but as engineers, it's so difficult to design for them." He explains that the city's traffic is predominately vehicular traffic, so timing all the traffic signals to account for that and a small amount of bicycle traffic increases delays, noise and air pollution.

Many cities like Pleasanton currently have embedded road sensors that detect both cars and bicycles. But Contra Costa Times mentions that if the bike isn't positioned properly or isn't made of metal, those sensors don't work. The performance and success rate of video detection, another widely used method, is affected by fog and wind. The microwave sensors' detection are not affected by weather, and are immune to post-rain glare, sunrise and sunset, according to the manufacturer.

The sensors cost about $4,000 and $5,000 each. The technology was first paid by Measure B dollars, compiled from an Alameda County half-cent sales tax for transportation projects, and from funding specifically identified for pedestrian and bicycle projects. An annual operation and replacement budget also helps replace some older technology, Pack says. The Intersector's cost aligns with the cost of past detection technologies, so when replaced over time, there's not really a cost increase. "We have to do detection at some point, whether we choose one technology over another," Pack says. "This isn't really more expensive than those other options."

Pack says he would ultimately like to deploy these devices at every signaled intersection, but that likely won't happen for another 10 to 15 years. Despite this, he's enthused that the city is making traveling safer for both motorists and bicyclists. "They're both fully legal on the road, they have equal rights," Pack says, "and now we're finally able to provide the service that they've deserved for so long."

Tina Trenkner is the Deputy Editor for She edits the Technology and Health newsletters.
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