Why Doesn’t the Light Turn Green for Bicyclists?

Why Doesn’t the Light Turn Green for Bicyclists?

One of the more frustrating aspects of cycling in an urban environment is not being able to trigger a signal and being stranded at an intersection. There you are, riding happily along, when you come to a red light and — after an entire signal cycle — you get the sinking feeling that the light is ignoring you. How do traffic light sensors work?  Is there a special technique for getting the green? 

In response to a question we received on the BikeLoudoun website regarding this problem, I contacted the Virginia Department of Vehicles Northern Virginia Bicycle and Pedestrian Program Coordinator, Fatemeh  Allahdoust, who forwarded my question to VDOT Regional Operations Maintenance Manager, Mark D. Hagan. The following is a detailed description of how traffic lights recognize vehicles, notably bicycles.

According to Mark, most of our traffic signals in Northern Virginia use what are known as "inductive loops,” which is a loop of wire located within the pavement with a small electrical charge that creates an electrical field. The electronic device within the control box cabinet looks for an inductance change within that electrical field and triggers a signal that a car, motorcycle or bike is present. Note that to create an inductance change within the field requires a metal alloy that is substantial enough to be recognized. These days, with advanced design and innovative compounds being used to produce motorcycles and bicycles, it is becoming increasingly difficult or impossible to recognize their presence. 

VDOT also utilizes a detector that is referred to as Video Detection (VID). This is a camera-based technology that is triggered by a change in pixel shades. The technology--more specifically the algorithm—looks for a percentage change in the pixel signatures of what the camera is looking at. This technology is not reliant on what the vehicle is made of, just how much change can be registered within the camera’s view. Unfortunately, this technology is greatly affected by rain, snow, fog & shadows.  Basically, it will detect almost anything that passes in front of the camera, making it unreliable for most applications.

The newest and most promising “out of pavement” or non-intrusive detection is what is known as microwave technology. In the past, microwave technology was not reliable for anything other than mid-corridor count data as the unit could not tell when a vehicle stopped or slowed to less than 5 mph. Because of this, it has never really been used for permanent traffic applications. However, due to advancements in this technology these devices are showing promising results at test locations, and do have the ability to recognize cars, motorcycles and bikes. Currently VDOT is looking at this technology’s ability to accurately count vehicles, and also to work in inclement weather.

Regarding the issue of WHERE bicyclists should stop in order to trip the light to green, the corner of loops can recognize bikes if they are made with enough metal (this goes back to the inductance issue mentioned above.)  Unfortunately, these in-pavement detectors are sometimes hard to locate by riders if the road has recently been overlaid with new asphalt.

The majority of VDOT traffic signals are outfitted with pedestrian buttons, and the bicyclist can trigger the green utilizing that method.   Other than that, Mark said that technology has just not kept up with demand, which is why VDOT engineers are closely looking at improving microwave detection.

According to Mark, new ground-breaking research on vehicle detection and tracking is being done at Federal Highway Administration (FHWA) research labs at Berkley & MIT. One avenue being explored would equip each vehicle with a device utilizing technologies like blue tooth, which will enable it to be tracked and recognized by the traffic signals. An actual emitter would be located on the vehicle or person (think of toll road emitters or GPS on telephones). The system ( i.e. the traffic signal), would be actuated by this technology. This would or could eliminate the need for in- or out-of-pavement detectors and eliminate motorcycle or bike non-detection. It is likely that the FHWA will support this study by funding pilot applications in select localities.