Sunday, November 9, 2008

Seeing red, part 3: Sensor-actuated signals

Take a look at the road surface when you approach a traffic signal. If you see a tar outline of a rectangle with cut-off corners, the road holds a coil of wire that acts as a metal detector to trigger the traffic signal. If a car stops on top of that metal detector (a.k.a. inductive loop sensor), the car will trigger the light to turn from red to green. But what if a bike stops on top of the sensor? Usually, nothing at all.

When an electrical engineer's explanation of bicycles and sensor-actuated traffic signals first appeared online in 2003, I brought it to the attention of Metro traffic officials. The article explains how to make the loops sensitive enough to detect bicycles without getting triggered by larger vehicles in adjacent lanes. It also tells how to mark streets to show bicyclists the "sweet spot" of the sensor. Several cities have made bicycle-sensitive loop detectors their standard. The Louisville Metro Complete Streets manual approved in 2007 mentions bicycle-sensitive signals once, but does not require or recommend making all sensor-actuated signals sensitive to bicycles.

How much of a problem is this? If you don't care about whether bicyclists stop at red lights, then you probably won't rank it high on your list of necessary improvements to our bicycling environment. I make a point of riding according to traffic law, and these bicycle-ignoring signals drive me crazy. Here's a short top-of-the-head list of signals that I can't trigger, no matter where I place my bicycle on the sensors: Spring Street at Payne Street; Payne Street at Baxter AvenueCountry Lane and Brownboro Road (in front of Doll's Market); North Bellaire Avenue at Frankfort Avenue; and Hillcrest Avenue at Frankfort Avenue. The light at Hillcrest and Frankfort is even more crazy-making, because it flips back from green to red 2 or 3 seconds after a motor vehicle gets off the sensor. A bicyclist waiting behind a motorist at this red light can make it through on green only by tailgating the motorist and sprinting.

I expect that Louisville Metro and the surrounding counties have hundreds of intersections controlled by sensor-actuated signals. Maybe half of these signals will trigger for a bicycle placed at exactly the right spot; the others won't trip for an individual bicycle, ever. These sensors usually get replaced whenever the road is repaved. It costs little to make the new detectors bicycle-sensitive. City, county, and state traffic engineers: Please build signalized intersections that work properly for bicyclists. How can you expect us to obey red lights that don't recognize our existence?


john said...

I've never been able to get the signal at Ewing and Frankfort to change for me on a bike. Interestingly enough, I believe this is part of a signed bike route.

purple haze said...

While not specifically "Louisville," one that frustrates me is the light for folks turning left from US31 Northbound toward the Colgate plant. That left turn shaves several turns and some time off my commute route in the mornings if I can catch a car making the turn (and thus tripping the light's signal). If no cars are there to help, I don't feel bad about stopping, then proceeding when it's clear my travels will not affect others' travels.