Warning: Technical bicycle content.

As an avid bicyclist, I’ve often wondered what these markings mean:

I’ve only ever seen this marking in San Francisco (although I am assured it exists elsewhere). I always assumed that it was a smaller version of the much-beloved Sharrow, which serves to remind motorists that bikes are legitimate sharers of the road (and, I think, to assure cyclists they are on a designated bike route):

It turns out, though, that it’s considerably more complicated. And possibly of practical interest to cyclists.

Many traffic lights in San Francisco actually detect when cars are waiting for the light. They use a technology called “inductive loop,” which uses magnetic fields to detect metal objects that are over the sensor. You’ve probably seen inductive loop sensors on the pavement. They look like a box sliced in the street. Something like the shapes in the foreground of this shot:

The thing is, these sensors are in general tuned to detect cars. And cars consist of a lot more metal than your average bike. Which means that the sensors have a much more difficult time detecting bikes. The sensors can detect bikes, though, which consist of just barely enough metal for the sensors to detect. According to a recent study (PDF), many bicycles are at the outer limit of what the sensors can deal with. So a bicyclist who wants to make sure to get a green light will want to stop her bike at the most sensitive part of the sensor.

Probably the most common type of loop in California is the 6′ square Type A. Type A loop will detect a bicycle, but only if the rider knows where to stop on the loop. Because a bicycle is vertical, the best place on the loop for a bicycle to be detected is on top of one of the sawcuts parallel to the direction of travel, as shown in Fig. 2.

EDIT: Apparently I was unclear. The little bike symbol marks the spot where you should stop your bike to have the best chance of being detected by the traffic sensor. Once you become familiar with where the little symbol is placed in relation to the cut marks on the street, you can stop your bike on the equivalent part of sensors where there’s not even a mark.

In other news:
A few people have chastised me for not blogging enough lately. Well, since we have had a rare light election year, I’ve been spending some time doing this: and unfortunately, it gets in the way of blogging. (You can find out more about cyclocross racing from an article in today’s New York Times) December will continue to be light for me, but look for renewed energy and perspective in the new year.

Sharrow pic from Richard Masoner, on whose blog I first saw the inductive loop info.
Sensor pic from Geoff McKim
Racing pic from Kelleigh Trowbridge