Inductive Vehicle Loop Detectors – Resources for good design and installation
One of the perks of employment as a civil engineer with a small governmental agency, is the opportunity to handle all sorts of projects from inception to completion. There is also plenty of opportunity to experiment, and explore options to increase efficiency and lower maintenance overhead in mundane, every day designs and tasks. After all, smaller agencies have less resources to begin with; why not use all available resources as efficiently as possible.
Our traffic signal maintenance forces were spending too much time and energy chasing loop detector related maintenance calls. The time had come to not only examine closely our standard detail and specifications for loop detectors, but those of other agencies around the country, and the world.
The following information is the summary many hours of research spent studying the best features of loop detector details from various public agencies, and construction of over 25,000 linear feet of loop detector, including over 500 Type E circular loops. Many thanks to some excellent, engaged discussion on the ITE Traffic Engineering Council listserv, including the expertise of Jim Harris, Rob Klug, and Mike Whiteaker.
These design techniques have shown to be effective in reducing loop detector related traffic signal maintenance calls within our agency:
1. Design adjacent loop detectors at oblique angles to each other, wherever possible. This practice ensures that magnetic field lines of adjacent detectors do not operate in the same direction, which reduces loop crosstalk and interference. A series of 6 foot (ft.) by 6 ft. diamond, or 6 ft. diameter circular loops in each lane is a method to accomplish this design. Circular loops also hold up better to freeze-thaw and shifting asphalt pavement. Oblique angled loops also detect bicycles better than loops which are sawcut parallel and perpendicular to the direction of travel. The article Anatomy of a Bicycle Friendly Street: Loop Detectors at LADOT Bike Blog has a really good explanation on how to reliably detect bicycles with loop detectors.
2. Always wire adjacent loops in opposite directions, as shown on page 122, figure 115 of the FHWA Traffic Detector Handbook. For example, the first and third loops in a single lane should be wound in a clockwise direction, and the second and fourth loops in the same lane should be wound in a counter-clockwise direction. Loops in adjacent lanes should also be wound in opposite directions. This will alternate the polarity of adjacent loops, which will further reduce crosstalk and interference. Twisting the home run wires of each loop a minimum of seven turns per ft. also reduces interference.
3. Label the start of each loop wire in the pull box, and connect it to the black wire of the loop lead going back to the controller cabinet. Land each pair of loop leads to the detector panel in the following sequence: black, white, black, white, etc. This will ensure the alternating polarity of the loop detectors in the road
4. NEVER wire multiple loop detectors in parallel on the same loop lead; ALWAYS wire multiple detectors in series only. This also applies to landing two loop leads to a single set of terminals at the cabinet, which is essentially the same thing. It is possible for a loop detector wired in parallel to fail and go unnoticed if a good loop detector is wired in the same circuit. For example, say the stop bar loop and an adjacent loop are wired in parallel, and the stop bar loop goes dead. A vehicle pulls up on the approach, but is only over the stop bar loop (which is bad) and not over the good loop in the circuit. The vehicle will not be detected, because the stop bar loop is dead, however, the channel on the loop amp WILL NOT fail open, since there is still a good loop in the circuit. This vehicular approach will only receive a green signal when another vehicle pulls over the good loop in the circuit.
5. Abandon old loop detectors properly by cutting them in at least two locations around the head of the loop. Loops not properly abandoned can act as an antenna and can cause interference to new loop detectors installed nearby.
6. Loop detector installation specifications should require IMSA Type 51-5-1984 #14 “loop-in-a-tube” wire, and IMSA Type 50-2-1984 #14 shielded leads. Loop-in-a-tube wire offers added protection to wires installed in the pavement, and shielded, twisted leads offer resistance to crosstalk and interference.
7. The number of turns of the loop wire does not increase loop sensitivity; it only increases efficiency of the detector. Most modern loop amplifiers from Reno A&E, GTT, EDI, and Naztec will work fine with 100 microhenries per loop, which equates to four turns for a 6 ft. by 6 ft. loop detector. If your lead-in cable is particularly long (greater than 300 ft.), then add an extra turn to improve the efficiency of the loop detector.
8. Quadrupole loops typically have half the detection height of rectangular loops of the same size. A 6’x25′ quadrupole loop is really two 3’x25′ loops side-by-side; consequently it has half of the detection height as a regular 6’x25′ rectangular loop. A series of 6 ft. by 6 ft. diamond, or 6 ft. diameter loops will detect high-body trucks at a lower sensitivity setting on the loop detector amplifier than a single long quadrupole loop. Another benefit to this design is if one of the loops in the series fails, it can be taken out of service and the phase placed in locking memory detection. Alternatively, a single long loop which has failed typically requires that the phase be placed in maximum recall until the loop can be replaced.
9. If wiring one loop per detection channel on the loop amplifier, try to design a series of 4 loops, as shown on the details referenced above. Loop amplifiers are typically manufactured in 2 or 4 channel models, and a series of 4 loops, each wired to an individual channel, will consume one 4-channel amp, or two 2-channel amps. Wiring one loop per detection channel will also allow for tuning of separate frequencies of adjacent loop detectors, thus minimizing crosstalk and interference.
10. Placing more than one pair of loop home runs in a single sawcut slot is okay, as long as the home runs are twisted at least 7 turns per foot. Installation crews should be encouraged to put no more than 2 loop home runs in a single slot, where possible, in order to minimize cutting the pavement, especially when loops are cut in the top. Contractors are typically paid per linear foot of sawcut; loop detector specifications and/or details may need to specify that payment will be made for the entire length of each loop home run in the slot, and not necessarily just the length of slot with multiple home runs.
By following the above items #1 thru #6, all interference and cross talk problems were virtually eliminated with loop detectors installed with this detail. Especially helpful is the Reno U1200 loop amplifier. It has an LCD display where the frequency of each loop amp channel can be visually determined. Adjacent loop channels can be conveniently tuned to be far enough apart in frequency from each other.
Below are resource files and links for loop detector design:
Burlington County traffic loop detector installation detail (Adobe PDF format)
Burlington County traffic loop detector installation detail (Microstation V8 DGN format)
Re-Evaluating Signal Detector Loops (Adobe PDF format)
Marsh Products Inductive Loop Write-up (Adobe PDF format)
CalTrans Electrical Detail ES-05a (Adobe PDF format)
CalTrans Electrical Detail ES-05b (Adobe PDF format)
FHWA Traffic Detector Handbook FHWA-IP-90-002
Inductive Loop Design Guide – EDI
Detection of Bicycles by Quadrupole Loops at Demand-Actuated Traffic Signals
SCDOT > Motorcycles & Traffic Signals > Loop Detectors
Traffic Signal Loop Detector Installation – Part 1 – from Trafdata.com
Traffic Signal Loop Detector Installation – Part 2 – from Trafdata.com