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.