stoplights

Yeah, I do a fair bit of urban cycling. I also do a little bit of blog reading, often focusing on other urban cyclists and cycling issues.

I do some group riding as well. I'm always surprised at the number of riders who are not aware of what I consider easily-accessible general information on some issues important to anyone riding (or even driving, for that matter) in an urban setting.

Take stop light technology, for example. I'm not talking about understanding all the science and programming inside their control boxes, nor that we've moved most of our signals to LEDs instead of incandescent bulbs (saving energy AND making the lights more visible). I'm talking about how those signals recognize our presence.

In the downtown grid, many of the lights are set on timers, and those timers are coordinated to allow traffic to move at a certain speed. Jefferson Street is an example--one can stand at the east end of Jefferson Street and watch the lights change in sequence over the course of a couple minutes.

Away from the "grid," though, many lights are triggered by sensors. Some of us remember the old weight-sensitive plates placed in the pavement a few feet or yards before the stop line (that wide white painted strip at the intersection). Those were not as reliable as the current (pardon the pun) system that uses properties of electromagnetism to detect vehicles waiting for the light to change (no moving parts in the newer system, for example).

Many of these traffic lights have quadripole sensors. I'm not an engineer, so I am not able to fully explain what makes them quadripole, but I know that they are easy to spot--they are the ones with the squared-off figure eight cuts in the pavement, as the following diagram shows.






The picture shows that the Public Works folks have painted a bicycle with rider on the sensor loops. This shows where the sensor is tuned to be more sensitive to the wheels of a bicycle. A cyclist can trigger the stop light to change at this spot on the pavement.

Barry did some blog posts on this topic some months ago, when he went out with Dirk Gowin and some Public Works folks to test a few traffic lights.

Cyclists are not the only ones who don't know this about traffic lights, though. I have often seen motorists sitting in an intersection, with their rear bumper over the stop line, wondering why the left turn light won't turn green for them... It's the sensor, dude. If you are not on it, the light doesn't know you're waiting!