Bicycle Adaptive Traffic Signal Timing

I’m only an occasional bicycle rider but I have always thought that it would be nice if traffic signals could respond appropriately to the special characteristics of bicycles. Bicycles are different from cars. Among other differences are bicycles do not have air bags nor 5 mph bumpers. Forgetting that though, there are other differences between bicycles and motorized vehicles that need to be addressed. The three most important differences are acceleration, speed and stopping distance. A motorized vehicle can do all three better.

Still, there are good reasons to ride a bicycle. Better fuel economy, good exercise, less pollution, etc. are three very good reasons for riding a bicycle, especially for shorter expeditions.

Many people opt to utilize their bicycles and current rules of the road allow a mix of bicycles and motorized vehicles on the same roads. For many years there were no special considerations given to bicycles. More recently, some signals have installed special bicycle push buttons to provide bicycle timing. This requires the button to be located adjacent to the bicycle path or for the bicyclist to go to the pushbutton sometimes requiring dismounting. Not altogether convenient.

Many agencies made their inductive loop detectors sensitive to bicycles. The MUTCD has even been adjusted identifying bicycle detection requirements. This allows the bicycle to be detected but the timing provided is usually set for motorized vehicles rather than bicycles. There are often also other problems encountered unless special loops are utilized. A loop designed to pickup bicycles often will not detect trucks or SUV and or may detect cars in adjacent lanes causing improper traffic signal timing.

Often, two detector systems are installed in the traffic lane, one to detect bicycles one to detect other vehicles. Obviously this can be a costly method of detection. There are two things that would make things better for bicycles without adversely affecting motorized vehicles. One is bicycle adaptive timing. Adjust the traffic signal green and/or clearance time to be appropriate for the slower bicycle. In order to do this, a good means of detecting bicycles needs to be utilized. It would be better if the same loop detector system could discriminate a bicycle from other vehicles either by the signature of the actuation, amount of actuation, or special loop location.

Obviously, the easiest way is providing a special lane for bicycles that cars don’t travel over. This can be as simple as installing loops in the bicycle lanes. This does require that there is room for the bicycle lane, that bicycles use it and that cars don’t travel over it such as when making right turns.

A better approach is loop detectors that discriminate bicycle actuations from other vehicles. This is possible by providing a different output when the frequency variation of the loop array is low and a separate output or output condition when the actuation is greater. A typical bicycle causes 10-16hz frequency variation while a small car would be over 20 hz variation. Unfortunately, some other considerations need to be made to prevent false bicycle actuations due to cars passing near the loop rather than over it.

A better method would be to utilize an actuation signal signature difference to identify and discriminate bicycles from other vehicles and then provide the individual outputs for each. This should just be an adaptation of the typical counting detector operation. These detectors utilize loop arrays and only provide a single pulse for each vehicle that leaves the loop array. They have accuracies of greater than 98% and should be capable of providing bicycle discrimination with appropriate programming.

OK, now assume we have appropriate detection. Now, we need to adjust the traffic signal timing for the bicycle while still providing the least impact to the other traffic. There are three considerations. A bicycle starts out and accelerates much more slowly than other vehicles, doesn’t go as fast, and doesn’t stop as quickly. A traffic signal typically provides a minimum green time, a green extension time and a clearance time appropriate for the motorized vehicles. If the traffic controller could provide a different minimum green, green extension and clearance time adjusted appropriately for the lower acceleration and speed of the bicycle. Existing formulas can identify appropriate intervals, the key is having a controller that is capable of timing the intervals.

For example, a typical vehicle minimum green might be 6 seconds, green extension might be 3-6 seconds and a clearance interval 4.5 seconds for a roadway with a speed of 35 mph. When adjusted for a bicycle, the minimum green might be 8-10 seconds, green extension 6-10 seconds and the clearance interval of 5-8 seconds.

An important note here is that the vehicle clearance interval consists of both the yellow interval and the all red interval. The yellow interval should not be extended for the bicycle. The all red interval should provide the additional clearance. This is because motorized vehicle traffic could have difficulty responding to varying yellow clearance intervals since most drivers consider it green time and proceed to travel through the intersection on the yellow.

The Naztec model 920 traffic controller provides such timing. Reno A&E evidently has a bicycle discriminating loop detector system available. I haven’t seen the detector yet, but I’m told it exists and is in use.

Obviously, if the discriminating loop detector works, it really opens up adaptive bicycle timing. Even if a controller didn’t have any special timing capabilities, the detector could be set to provide different extension intervals for the two outputs allowing at least a bicycle appropriate green extension interval.

The real benefit of having a controller with bicycle timing capabilities is the ability to provide the extended clearance time for bicycles. At larger intersections, left turn travel distance can reach up to 180 feet. The amount of time it takes a bicycle to travel that distance could be more than 15 seconds. Typically, the motorized vehicle travels the distance in less than 6 seconds. Obviously, good practice doesn’t allow always having the longer interval for bicycles when they are not present.

To sumarize bicycles and other vehicles are different. Different timing needs exist for each. Methods now exist to provide for those differences.

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This is the first draft of this document and will probably be updated as errors and ommissions are identified. If you identify errors or can provide additional information please contact the editor using the contact form on the right sidebar or register and post a comment directly to this article.