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.
Relevant searching on Google, along with some excellent, engaged discussion on the ITE Traffic Engineering Council listserv, produced the following the design techniques which 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, and consequently, reduces loop crosstalk and interference. A series of 6 foot (ft.) by 6 ft. diamond, or 6 ft. diameter circle loops in each lane is a method to accomplish this design. Oblique angled loops also detect bicycles better than loops 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. Quadrupole loops typically have half the detection height of rectangular loops of the same size. 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.
3. 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.
4. Label and connect the start of each loop to the black wire of the loop lead and 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 loops in the road.
5. 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.
6. Never wire multiple loop detectors in parallel on the same loop lead; wire multiple detectors in series only. 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.
7. 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.
8. 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.
9. 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.
Below are resource files and links for loop detector design: