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.
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.
References:
- Traffic Signal Bicycle Detection Study: Final Report, City of San
Diego, 1985
- Bicycle Forum Tech Note F-2: Bicycles and Traffic Detectors; John
Williams, Bikecentennial, 1990
Topics for further study:
- Quadracircle loop detectors
- Video camera detector systems
- Loop locations on the roadway
- Loop marking locations
- Pros and cons of push buttons
- Signal detection awareness campaigns
Options: