Most of the sales reps in my area have figured out that I like technology. I am willing to put technology in place in intersections. One of the new buzzwords in traffic signals is radar detection. Even better, the neat new technology is combining radar and video detection in one device. There have been multiple generations of radar and video detection. Some have worked really well. Some, not so much.
This post is not attempting to sell a product, rather a general discussion of what why some technologies work well beyond the technical specifications.
I currently deploy video detection and radar detection for count stations, stopbar detection and advanced detection. It is true that nonintrusive detection (radar, video detection etc) cost more to implement than traditional loops. There are several advantages to nonintrusive detection. These include, but are not limited to increased flexibility, immediate gratification, long term cost savings, ease of repair and other factors.
Flexibility
The easiest way to envision flexibility is that someone always has the idea that they should restripe the road. Move the lanes around to create a bike lane, squeeze in a turn pocket, repurpose an approach to create dual lefts, or some other operational modification. The non-intrusive detection allows for a decreased cost for reallocating the detection to the new lane configurations. If the vehicle detection is all induction loops embedded in the pavement, then someone has to pay for the traffic control and labor to cut in the new induction loops in the new locations.
One particularly troublesome intersection that vexes me has no detection at all for a left turn pocket with a protected left turn (it was cut out a long while ago and never replaced), and another approach where the lane lines were restriped that ended up with the old left turn lane loop detection still being used, but it was on the lane line separating the left and thru lane. If this had been video or radar detection, these zones could easily be reprogrammed in the detection units.
For some reason, engineers have designed lots of bike lanes, and in many cases, have no detection in the bike lanes. Worse yet, in too many cases, the bike lane detectors are configured badly, where you have a small (I have seen 2-ft wide by 10-ft long quadrapoles as bike detection) loops in the bike lane at the stopbar.
Several states now require that traffic signals detect all legal vehicles. A well designed bike loop can detect a steel bike (through the ferrous metal affecting the inductance field) or an aluminum bike (where the aluminum causes an eddy current in the loop that causes a change in the inductance field), well designed loops have problems at best sensing a carbon fiber bike. By extending the radar stopbar or video detection zones into the bike lane, the bike is sensed by the signal.
Immediate Gratification
There appear to be a dearth of loop cutting contractors. Regularly, traffic signal turn ons occur where the signal is ready to be turned on today, but the loop cutters have not been scheduled yet. It is not unusual for a signal to be turned on, and operated in pretimed mode for an extended period of time before the loops get cut in and terminated. In many cases for developments, the requirement is for the signal to be fully functional, but the political decision is made that getting the signal turned on, going green, yellow and red is good enough. The opening of the development can occur even though the signal is not fully functional. Once the developer has the occupancy permit, the traffic engineer has no hammer to get the loops cut in. The developer is usually capable of making the straight faced argument to the elected officials that the loop cutter is just a short time away, so, please let the business open. I understand the argument by the developer. They have a significant capital investment in the property. The business has a significant capital investment in getting to the point where they are ready to open. The loops can’t be cut in until the asphalt is placed… The asphalt is placed two days before opening, then comes striping… It was wet over the weekend, so the asphalt is late, the striping is late… loops get the short shrift.
I have had traffic signals that have operated in pretimed mode for 3 months or more, between when the signal was turned on, and when the loops were finally installed and terminated.
By going to video or radar detection, all of the vehicle detection installation can take place when the poles are erected, weeks to months before the opening day. We work with the contractor to aim the cameras and radar detection panels early. In many cases, we go out with a generator and set up the video or radar detection systems after the cabinet is installed and wiring is pulled and terminated, but before the electrical service is connected to the transformer. That means on the day of turn on, all we are doing is refining the detection zones.
Another form of immediate gratification is where utility work cuts through loops, or loop lead ins. We had an intersection where a water line blew out immediately below the southbound stopbar loops for a major intersection. In order to repair the water line, the stopbar loops for all 4 lanes of the southbound approach had to be cut. Between the immediate utility work, patching, then removal of patching, and replacement of the pavement, the loops were reinstalled about 2 months after the water line blew out. During that entire 2-month period the signal was in recall for the left and thru lanes, 24 hours a day. It is pretty straightforward to take a coordinated traffic signal, and put the main street coord phase into recall. During the hours of the day that coordination is in effect, the drivers will most likely not see any difference in the signal operation for that particular movement. During the non-coordinated hours of operation, this will make the signal very sluggish.
The left turn lane is a different thing. Putting a left turn lane into recall for 2 months is painful, especially when the opposing thru is very heavy.
We have Ethernet communications to all of our signals that have video and radar detection. Where we have had utility work at signals with radar and video detection, we are able to quickly modify the detection zones, and observe that the changes are working well.
Long Term Cost Savings
Video and radar detection systems likely cost more than initial installations of induction loops. However, over the long term, the nonintrusive vehicle detection systems will pay for themselves.
Grind and Overlay
There are several methods to maintain the pavement surface. When the decision to grind and overlay the existing asphalt this can be accomplished several ways. One way is to grind out the entire pavement surface, the other is to retain as much of the pavement surface as possible, but to grind out a wedge of asphalt approximately 2-inches deep at the curbline, and taper to no grinding at 10 to 12-ft from the curb. This second method is quite common where the pavement is in relatively good shape, but the desire is to keep the top of the curb at 6-inches above the gutter elevation. It is not uncommon for a paving project to include significant patching, where small to large sections of the pavement are completely cut out to below the subgrade, and replaced full depth.
The specifics of the pavement replacement method is not really important to the loops. What is important to understand is that even if the standard plan shows that the loop wires be at 3-inches below existing surface of the pavement, when the top 2 inches of asphalt are ground out, the grinding will most likely pull up the sealant, which is bonded to the loop wires running from the loop to the junction box. When the sealant, which is bonded to the loop wires is pulled up, the pavement grinder will shred the loop wires, typically between 4 and 1 feet from the gutter line – where the grinding includes a wedge of asphalt removal.
As an aside, it is important to understand that if loops are replaced above the existing loops, the old loops will create false calls if the old loops are not dealt with properly. If the old loop is not fully cut in multiple locations, then the shredded loop wires near the gutter line will allow the old loop to couple and decouple below the new loops, which will cause false calls on the new loop. This will occur most frequently when the pavement gets wet.
On one grind and overlay project I worked on, the grinding took out the main street loops on a 5-lane Principal Arterial at 6 traffic signals. Replacing the loops at these 6 traffic signals cost over $100,000. All of the existing loops worked properly on that corridor, but when the asphalt wedge was ground out, the grinding shredded every loop wire at or near the gutter line. The loops themselves were not affected, just the wires between the loops and the splices in the junction boxes.
Four years after the loops were installed due to the grind and overlay, the same road is going to have a sanitary sewer trunk line installed, which will require replacement of most of the loops… again. If the vehicle detection systems had been video, or radar, instead of loops, the initial grind and overlay, and then the upcoming sewer work would have not required the replacement of all those loops, and the cost associated with replacing the loops.
Underground Utility Work
Recently, I was approached by the local sewer utility, where they had four locations where minor sewer work needed to be accomplished. Each of the four locations had 200-ft or less of sanitary sewer to be installed. The problem was, that in each of the four locations, the sanitary sewer work was going right through a bunch of stopbar loops. Each location was going to require between $4,000 and $6,000 worth of loop replacement. In each case, the cost of the loop replacement will need to be passed on to the sewer rate payers.
Once again, if the vehicle detection systems were radar or video, instead of loops, the cost of replacing the vehicle detection would not be borne by the citizens.
Ease of Repair
When a loop shows problems, it could be any number of issues. The problem could be any combination of one or more of the following: the loop itself has gone bad, the splice between the loop and loop lead in wires is bad, the loop lead in wire is bad, the field wires are not firmly connected to the terminal screws in the cabinet, the loop amplifier has gone bad…
There is a similar laundry list of things that can go bad for video or radar detection.
The ease of repair comes from replacing an induction loop that has gone bad in the street, compared to replacing a camera or radar panel. To replace a camera or radar panel, the detector needs to be replaced in the same location, usually with a bucket truck. This likely requires that one lane be coned off for a relatively short period of time to replace the bad detector with a new detector.
When a loop, or series of loops goes bad, there needs to be multiple vehicles, equipment, and personnel out in the street for extended periods of time. This causes one or more lanes of traffic to be routed around. The wire from the loop to the junction box needs to be cut and laid across multiple lanes of traffic, which requires multiple sets and resets of traffic control. Work needs to be done at the curb, for where the loop’s sawcut goes into the conduit that leads to the junction box. There is a great deal of risk associated with all of this activity in the street.
Going from an in-pavement detection system to an out of pavement detection system reduces the risk associated with traffic maintenance operations.
Conclusion:
Induction loops can be more accurate than video or radar detection. There are short term and long term benefits to seeking other methods of vehicle detection than induction loops.
In a later post I will discuss the benefits of different types of radar detection that far exceed the ability of induction loops.
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