FAQ Rec.Bicycles Frequently Asked Questions Posting Part 4/5 Traffic Detector Loops

A traffic loop detects metal objects such as cars and bicycles based on the change in inductance that they induce in the loop. The loop is an inductor in an LC circuit that is tuned to resonate at a certain frequency. A metal plate over the loop (like a car) causes the magnetic flux to be shorted, reducing the inductance of the loop. This causes a change in resonant frequency, which is detected and sent to the signal controller. One of the ways of testing a loop is to create a loop about 2 feet in diameter with several turns of wire (connecting the ends) and placing the test wire in the middle of the traffic loop. The test wire should cause a detection, if all is working.

The same effect is seen with a vertical piece of metal, such as a bicycle, but is weaker. Because aluminum conducts electricity quite well, aluminum rims help. Steel rims are OK. Non-metal rims cannot be picked up at all. A bicycle with aluminum rims will cause about 1/100 the change in inductance of a car.

It is always possible to set a detector's sensitivity to pick up a bicycle. The trade-off is in longer detection times and the possibility of false detections from vehicles in adjacent lanes. Most people who set signal detectors use the lowest sensitivity setting that will pick up cars reliably.

I advocate using the highest setting that will avoid picking up vehicles in adjacent lanes. Digital circuits used in modern detectors can use high sensitivity settings without unacceptable increases in detection times. Unfortunately, there are still a lot of old detectors out there, and most people who work on signals use principles based on the performance characteristics of old detectors.

In any case, bicyclists should, as a general rule, place their wheels over one of the slots to maximize their chance of being detected. That is where the magnetic field perpendicular to the wheels is strongest. Bouncing the bike or moving it back and forth does no good. If you have a metal frame, another tactic that may work is to lay the bicycle down horizontally inside the loop until the light turns green.

Advancements are under way that may make traffic loops obsolete some day. In particular, radar, infrared and sound detectors have been introduced. Systems based on video cameras are especially promising. Such systems can easily detect bicycles. Such a system may even be able to detect pedestrians some day.

Merging into a bike lane is not the same as turning across it

In a recent column in Oregon Cycling, Bikes & The Law: The Right Hook, attorney Ray Thomas states, "the law clearly requires motor vehicles to first yield the right-of-way to bicyclists occupying the bike lane, just as vehicles changing lanes on a multi-lane roadway must first yield the right-of-way to other vehicles occupying the lane the driver would like to enter."

Not quite. Merging into a lane (changing lanes) is not the same as turning across a bike lane. A driver merges into a lane with other drivers going the same direction. A driver turning across a bike lane is crossing the bike lane, not merging into it. As the driver turns across the bike lane, in fact, he is going in a different direction from through bicyclists in the bike lane.

There is no name in traffic engineering for the act of crossing a through lane to make a turn, because such an act violates the principle of positioning your vehicle before you make a turn. Yet that is exactly what proponents of the Portland bike lanes are saying they want to happen. They want through bicyclists to keep to the right of right turning vehicles. I was taught never to pass a right turning car on the right during a group ride when I first started to ride seriously over 30 years ago. A little while later I was involved in the development of the bike lane law in California that was passed in 1976. We specifically designed that law to try to prevent right hook accidents.

On a freeway, slower traffic is supposed to keep to the right. But exits are located on the right side. Does that mean that a fast driver is supposed to take an exit directly from the left lane? No, you can get a ticket for that. Instead, a fast driver is required to merge into the right lane first, then exit, even if he has to slow down to match the speed of traffic in the right lane. That is exactly what the California bike lane law requires of drivers turning right from a street with a bike lane.

From what I can tell, both bicyclists who were killed recently in right hook accidents in Portland had pulled up to a red light next to a stopped truck. They were following what I have been told is the bike lane law in Oregon, which invites such right hook accidents. The Oregon law would be like expecting fast drivers on a freeway to exit directly from the left lane, being careful to yield to drivers in the right lane. Such an expectation is clearly unrealistic, so it is not allowed. Why should we expect a similar maneuver on a street with bike lanes to be reasonable or safe?

Mr Thomas also states, "if bicycles in bike lanes weighed the same as locomotives on railroad tracks (where the legal right-of-way principles are quite similar) there would be fewer motorists cutting us off because the result would be catastrophic for the motorist." When railroad tracks run parallel to a highway, the resulting highway/railroad grade crossings and highway/highway intersections are handled in a special way, usually with traffic signals. See the Manual on Uniform Traffic Control Devices.

Light rail lines that run in highway medians have a similar problem. Light rail trains are heavier than cars, and drivers turn left into the paths of such trains way too frequently. So it is not the weight of the train that makes the difference, it is the driver's expectation that they are not turning across the path of through traffic. The same principle applies to bike lanes to the right of right turning cars. It is the fact that bike lanes to the right of right turning vehicles violate a basic principle of traffic and thus violate a driver's expectation that is the problem, not the failure of such drivers to yield to bicyclists in the bike lane.

Report on my Bicycle Detection presentation to Caltrans HQ Electrical Systems Branch

On October 16, 2007, I gave a presentation, "Detecting Bicycles and Motor Vehicles Using the Same Loop Detector" to 3 members of Caltrans HQ Electrical Systems Branch: Theresa Gabriel, the head of the branch; David Maurin and Bashir Choudry. I recently sent most of you an article with what I intended to present, asking for comments. Thank you to those of you who provided input; it was quite valuable.

After my presentation, Ms. Gabriel responded to my recommendations for changes in the Caltrans Standard Plans, Standard Specifications, Transportation Electrical Equipment Specifications, and the California MUTCD that would address problems that bicyclists are having being detected at actuated traffic signals in the State.

Here are my recommendations followed by Ms. Gabriel's responses:

1. Configure the head loop as a diagonal quadrupole

Caltrans has no interest in investing any additional time or money into improving loop detection. Loops are an obsolete technology that are not reliable for the detection of bicyclists. Furthermore, Construction and Maintenance personnel at Caltrans complain that loops result in premature pavement failure, particularly in asphalt concrete pavement. Therefore Caltrans is looking to non-invasive technologies for detection at actuated traffic signals. Caltrans has contracted with PATH at the University of California, Berkeley, to develop this new non-invasive technology. Any new detection method, however, would need to be compatible with Type 170 controllers (and presumably 332 cabinets) and be able to hold the call as long as the bicyclist was stopped.

Caltrans is interested in bicycle detection and Ms. Gabriel said that she had been actively involved in the development of AB 1581 as well as unsuccessful versions of the bill in earlier legislative sessions.

Caltrans also has problems with insufficient funding to maintain the approximately 5000 traffic signals that it currently has. Caltrans cannot afford to upgrade its traffic signals to meet ADA standards, much less to detect bicyclists. Currently, Caltrans will install one or more Type D bicycle loops at a new or rebuilt traffic signal only if bicycle traffic is anticipated. But even when a Type D loop is installed they still receive complaints from bicyclists. Only bicycle pushbuttons are guaranteed, and they are not practical. Any new detection method will need to guarantee that all bicyclists are detected, including those who are riding non-metal bicycles or from children whose bicycles have small wheels.

2. Locate the loop where bicyclists are expected to stop, but if this is not possible, use a Bicycle Detector Symbol

Ms. Gabriel feared that making a bicycle loop larger in order to locate it where a bicyclist is expected to stop would result in detection of vehicles in the adjacent lane and crosstalk with loops in the adjacent lane. Caltrans is not interested in investing any funds into finding out whether larger bicycle loops have the problems that she fears.

Even with a Type D loop, bicycle detection is not guaranteed to be reliable. Caltrans is unwilling to use Bicycle Detector Symbols unless reliability can be guaranteed, because if a bicyclist stops over a Bicycle Detector Symbol and is not detected and proceeds on a red signal and is injured or killed in an accident, then Caltrans could be held liable.

Besides, the decision to install Bicycle Detector Symbols at actuated traffic signals and whether to add Bicycle Detector Symbols to the drawing in the California MUTCD showing bicycle loop locations lies with the Striping Branch within Traffic Operations, not the Electrical Systems Branch.

3. Use elastomeric loop sealant (or perhaps hot-melt rubberized sealant)

Ms. Gabriel said that the major cause of loop failures was pavement failure, not loop sealant failure as I had stated.

Caltrans has stopped using asphalt emulsion and epoxy loop sealant. It used to supply a 2-part elastomeric loop sealant to construction contractors, but now specifies only hot-melt rubberized asphalt. Although 3M loop sealant is a 1-part sealant and is likely considered elastomeric, Caltrans is unwilling to specify products that are only available from a single source.

Besides, the Transportation Laboratory is in charge of the loop sealant Standard Specifications, not the Electrical Systems Branch.

4. Connect the loops in series

Caltrans uses parallel-series connections not to reduce the inductance at the input into the loop sensor (as I had speculated) but to eliminate the need for a trouble call when a single loop fails out of a group of four. Given the lack of funds, however, Caltrans cannot afford enough traffic signal technicians to perform routine maintenance on detector loops or the equipment inside signal cabinets, so the failed loop is usually found only as a result of a complaint. The only routine maintenance that Caltrans signal technicians perform is to ensure that all signal indications are actually working.

5. ΔL sensor units preferable over ΔL/L sensor units

Such a change would require further development into loop detection, in which Caltrans has no interest.

6. Best to use a separate sensor unit for bicycle loop, but if this is not possible, add only quadrupole loops

Adding quadrupole loops rather than square Type A loops on a loop retrofit would require additional sawcuts, which Caltrans is not willing to do because of potential pavement damage.

7. Set the sensitivity of the sensor unit at the highest setting that will detect bicycles and still reject vehicles in adjacent lanes

Increasing the sensitivity setting increases the risk of crosstalk and of detecting vehicles in the adjacent lane. Using a bicycle or bicycle rim while setting the sensitivity would require closing the lane, which oftentimes is impractical because of heavy high speed traffic.
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I plan on helping with the development of new non-invasive detection methods that will reliably detect bicyclists. But I am still of the opinion that loops will be around for a long time, so I also plan on assisting local agencies in improving their detection of bicyclists.

Re: Caltrans response to questions about bicycle detection

A followup to my previous post:

On December 18, 2007, I had a telephone conversation with Jeff McRae, Chief of Caltrans' Office of Intelligent Transportation Systems Projects and Standards. He and a staff member of his wrote the letter that I included in my previous post. Here is the gist of our conversation:

1. I told him that the #1 problem that bicyclists have at traffic actuated intersections is being detected in the first place.

2. He said that Type D loops detect bicycles but that Caltrans is moving away from in-pavement detection because of safety and operational concerns.

3. I asked if he had read my presentation to the Electrical Systems Branch.

4. He said that he had read my presentation some time ago.

5. I asked him if Theresa Gabriel, head of the Electrical Systems Branch, had informed him that the Districts were telling her that bicyclists were complaining of not being detected in lanes equipped with Type D loops.

6. He said no, he was not aware of such complaints.

7. I told him that Type D loops are only 6' wide, the same as the square Type A loops that were introduced in the 1960's, so bicyclists who stop in the right side of the lane are outside of the loop and thus cannot detected.

8. I asked him if he knew George Palm, who I understand invented the quadrupole loop configuration (what Caltrans calls a Type Q loop). I told him that George worked for 3M and Canoga Controls but is now retired and that I had recently spoken with him.

9. Mr. McRae said no, that he was unaware of George Palm.

10. I said that George had taught that Type A loops needed to be located at least 3 feet from the lane line in order to reject vehicles in the adjacent lane. Since lanes are usually 12 feet wide, that led to the Type A loop being 6 feet wide. Caltrans has no records on the development of the Type D loop, so no one knows for sure why the developers of the Type D loop decided on a width of 6 feet. Most likely they selected 6 feet because that matched the width of the existing Type A loop.

11. I said that the big advantage of a quadrupole loop was that its magnetic field drops off much faster outside the loop than a conventional dipole (square, round or diamond) loop. That means that the sensitivity of the vehicle detector (Caltrans calls it a loop sensor) can be turned up to pick up bicycles and high-bodied trucks while still rejecting vehicles in the adjacent lane. The implication is that a quadrupole loop can be built wider than 6 feet and still reject vehicles in the adjacent lane.

12. Mr. McRae said that he had talked with experts on loops and that he understood that Type D loops larger than 6 feet wide would have a lower sensitivity for bicycles and be more subject to picking up vehicles in the adjacent lane.

13. I said that bicycles and motorcycles are like vertical plates of conducting metal and thus are detected if they stop on top of the wire in a loop slot and that they cannot be detected if they stop in the center of a Type A, B or E loop. Type D loops are diagonal quadrupoles and thus can detect a vertical metal plate anywhere within the boundaries of the loop. There are other possible configurations for a diagonal quadrupole than the Type D loop, which I discuss in my presentation.

14. He seemed to have a limited understanding of the physical principles involved in detecting bicycles with inductive loops, so I told him about my background of 3 years of electrical engineering at San Jose State before getting my BS in physics and then my MS and PhD in transportation engineering and subsequently specializing in traffic signals and bicycles. All four areas of my education, training and experience come together in helping me understand detecting bicycles with inductive loops.

15. He expressed concern that bicycles made of non-conducting materials are becoming more common.

16. I said that I am trying to spread the word that bicyclists whose bicycles are made of non-conducting material such as carbon fiber and who use them on the road for training or for transportation need to wrap a few turns of magnet wire around their rim, splice the ends and insulate the splice before installing the rim strip, tube and tire. He did not seem to follow how a bicycle wheel is made, so I asked if he was a bicyclist.

17. He said that he was not a bicyclist and that the information I gave him on how bicycle wheels are made was new. He also was not aware of what magnet wire is.

18. He said that Caltrans has decided to install Type D loops at all new and modified actuated traffic signals.

19. I said that if a bicyclist cannot see the Type D loop because it is covered over with asphalt pavement, then the knowledgeable rider will assume the worst case, which is that the loop is not a Type D loop and that he/she will have to stop the bicycle over the top of the loop slot containing the wire. But if he/she guesses wrong about where the slot is, then the bicycle will be out of the zone of detection and not be detected.

20. I said that one way to address the issue is to increase the zone of detection by making the Type D loop larger, decreasing the dead zone outside the loop. I said that the magnetic field of Type D loops, like Type Q (quadrupole) loops, decreases very quickly outside the loop, allowing the loop to be made larger and still reject vehicles in the adjacent lane.

21. He expressed doubt that a larger Type D loop would detect bicycles and still reject vehicles in the adjacent lane.

22. I said that I had asked for the documentation on the development of the Type D loop and that Caltrans had responded that it could find none. So there is no evidence to support his contention that larger Type D loops would be less sensitive to bicycles and more subject to picking up vehicles in the adjacent lane.

23. He said that although I was recommending larger Type D loops as a way of making the dead zone smaller, no matter how large a Type D loop is, it cannot cover the entire lane because of the necessity to avoid adjacent lane pickup. He then suggested that in my presentation to the CTCDC in Thousand Oaks on January 31, 2008, I focus on markings that tell bicyclists where to stop in order to be detected.

24. I agreed, telling him that one of the recommendations in my presentation was bicycle detector symbols for loops that cannot be seen I asked if he would be attending the CTCDC meeting in January.

25. He said no, he would not be attending the CTCDC meeting, but that one of his staff members would be there.

26. I told him that when I requested District 05, which contains Monterey County, to paint bicycle detector symbols at some nearby state owned and operated actuated traffic signals, their electrical engineer had replied no, out of concerns for safety of their crews and because of the cost.

27. He said that Caltrans is very concerned about both safety of their crews and cost, and that is why they are focusing on video detection.

28. I said that video detection is still rare at Caltrans signals, and that Caltrans owns and maintains thousands of traffic actuated signals with tens of thousands of loops. So any change to video detection is years into the future.

29. I told him that a Caltrans standard specification calls for the final lift of asphalt pavement be placed after the loops are installed. I understand that the reason for the specification is so that loop sealant on the pavement surface has a tendency to fail. I said that in my presentation I recommend the use of better loop sealants than the asphaltic material they are now using.

30. I said that at new or modified actuated traffic signals, the lane is already closed, so the placement of bicycle detector symbols can be done with no increased risk to their crews and little additional cost. If the final lift is to be placed on top of the loop, then the crew simply needs to note the location of the loop so it can paint the bicycle detector symbol after the loop is no longer visible.

31. He agreed that placing bicycle detector symbols during construction is simple. With that, he said he had another matter to attend to and signed off.