copyright 1996, Tracy-Williams Consulting

Traffic signals should work for bicyclists as they work for all other vehicle drivers. The trouble is that many don't.

How do most signals work?

There are two primary types of traffic signal systems: (1) pre-timed; and (2) demand-actuated. Pre-timed signals turn green through a set rotation; in other words, if you wait through the cycle, you'll eventually get a green light. These signals aren't too much of a problem for bicyclists. Demand-actuated signals, on the other hand, turn green when the system detects traffic. The problem is that many such systems detect cars and trucks just fine but don't detect bicycles well at all.

In general, demand-actuated signals consist of an electrified loop of wire buried in the traffic lane approaches to the intersection, a sending unit behind the curb, a controller box on one of the signal poles, and the signals themselves. When a large mass of metal moves over the top of the buried loop of wire, it alters the electromagnetic field which tells the sending unit to tell the controller to change the signal to green. This is what happens when a car arrives at the intersection. And it's why the signal can be red for hours until one car comes and it suddenly turns green.

How do you make signals work for bikes?

As you might imagine, most bikes don't have enough metal to make a difference to a loop detector. However, that isn't always the case. If, for example, the loop has a special shape, it's more likely to detect a bike. Or, if the bike stops in just the right spot (i.e., over the buried wire), it just might get detected. The photo above shows a special loop installation with a marking that was done in Boulder, Colorado.

Good loops: Many traffic engineers have experimented with bike-sensitive loops and they've come up with designs that work best in different situations.
  • Bike lane loop detectors often do best with "Quadrupole" loops, which look like figure-8s set on their side. These are particularly sensitive over the center and they lose sensitivity to the outside.
  • Traffic lane loop detectors often do best with "Diagonal Quadrupole" loops, which look like a figure-8 set at 45 degrees. These are sensitive over the whole width and allow bike detection even if you can't predict exactly where the bike will be. [The two loops in the above photo are diagonal quadrupoles.]
  • Changing local standards to use these loops would, over time, make a big difference in how bicyclists are treated by signal systems. The diagram below shows some popular loop designs.

    Marking existing loops: Standard loops are rectangular or square in shape. They are somewhat sensitive over the area in the center, most sensitive over the outer wires, and least sensitive outside of the loop. As a result, a bicyclist stopping at the curb would probably be outside the loop and would go undetected. No green light.

    On the other hand, a bicyclist stopping right over the wire has the best chance of being detected. Unfortunately, it's often difficult or impossible to tell where that wire is. But if you were to paint a pavement marking right over it, the mystery would be solved. The bicyclist could stop there and get the green light.

    The following diagram shows a fairly typical pavement marking used over the wires of a loop detector.

    What are the benefits?

    The primary benefits of making loops work for bikes are (1) to allow them to safely cross signalized intersections and (2) to send bicyclists the right message: they count as part of the traffic scene. It's a bit hypocritical to tell bicyclists they should obey traffic signals if we don't make them work for bikes.


    Topics for further study:


  • Return to System-wide Improvements
  • Return to Planning and Design
  • Return to Home Page