Road sensors not as simple as they seem
By Steve Wallace, Times Colonist, October 12th, 2012
There were so many responses to last week’s column about road sensors, it’s worth another look.
The most interesting information came from Rory, an electrician who installs the loop sensors. He has seen drivers actually bounce their cars in an attempt to register their presence. The old sensors were weight-sensitive. The new technology is different: Magnets in the newer looped sensors identify the metal of the vehicle covering them.
Most of the time, the loops are easy to spot. They are cuts in the pavement that have been filled with a tar substance that bonds with the road surface. Some loops are embedded in pavement upon initial construction, however, and are not easily identified.
Rory wants cyclists, motorcyclists and scooter operators to know a few features of the marked looping, whether circular, diamond or rectangular.
The best place to stop these types of vehicles is over the cut marks on the side of the loop. He does not recommend placing such vehicles directly over top of the looped shape. More metal, namely iron, will be exposed to the magnets if the smaller vehicle is positioned over the cut marks.
While it’s possible to increase the sensitivity of the equipment, there is a limit – if it’s set too high to accommodate bicycles, Rory says, the loop may detect a car in the adjacent lane, triggering an unnecessary traffic-light delay.
The sensors do not always detect bikes, especially those with aluminum construction. Drivers of smaller scooters are in the same situation.
The sensors are installed to eliminate unnecessary cyclical stops of through traffic, where less-used side streets intersect major roads or highways. They are also strategically placed to identify left-turn-lane traffic at major intersections. Again, the goal is to ensure through traffic is not held up unless a driver wants to turn left.
The sensors are usually placed immediately prior to crosswalk lines, where vehicles would normally stop while waiting for a traffic light. Sometimes, they’re placed well back of a particularly busy intersection.
When vehicle traffic stacks up for what seems to be an eternity, the sensors will send a message to the traffic light, resulting is a cycle change in direct proportion to the length of the line of waiting vehicles.
Ross from Victoria wrote to say he would appreciate a sign indicating looping sensor-technology use at specific intersections, a very good idea indeed.
Carol-Anne was somewhat indignant at being classed among “clueless drivers” who aren’t aware of the sensors. She asked a logical question: How is she supposed to know about them? Who is responsible for telling her?
Mic and Joe asked for pictures and diagrams of not only the sensor loops but other confusing driving situations. Although I have no control of pictures used in this column, I will endeavour to be more accommodating with the use of diagrams.
John, who regularly corresponds with me on driving matters, says there are references to the looping sensor in the road-safety guide for new drivers. He also reminded me that anytime a change takes place at a lighted intersection, a notice is posted. He rides a bike and maintains being perfectly still and properly positioned will trip the sensor.
Thanks for all the feed ack.
Steve Wallace is the owner of Joan Wallace Driving School on Vancouver Island and in the Central Interior of B.C. He is the former vice-president of the Associated Driving Schools of the Americas and a certified B.C. teacher.