Traffic loop detectors are one of the most popular types of traffic sensors. They are reliable, long-lasting, and automate a large percentage of traffic control at stop lights and intersections.
They consist of a saw cut slot in the road surface with multiple turns of wire that can be connected to an electronic amplifier. When a vehicle passes over the loop, it causes a disturbance in the magnetic field that is detected by the amplifier.
Installation
Traffic Loop Contractors in the road and detect large metal objects such as cars. When a car enters an intersection, it creates a disturbance in the magnetic field of the loop that tells the signal controller there are vehicles waiting to proceed through the light.
These devices work very well, but the results are only accurate if the pavement is not severely deteriorated and installation was done correctly. Poor pavement, street repair and improper maintenance can all degrade the effectiveness of traffic loops.
To improve accuracy, some agencies use portable loops that are preformed in a factory and sealed with an epoxy that is resistant to moisture and other chemicals found in typical sunbelt streets. These can be moved and reinstalled by one man and provide five feet of protected lead-in wire. Tests have shown that they perform well on a concrete surface and are expected to survive far beyond a million activations.
Repair
Inductive loops provide accurate vehicle detection for traffic signals but require digging up the roadway and lane closures. Over time, this leads to high maintenance costs and decreased performance.
The main cause of loop failure is pavement deterioration and improper installation. A poorly constructed sawcut or the incorrect number of turns in a preformed loop can lead to wire damage and false detection of vehicles outside the detection zone.
Another frequent failure point is the splice connecting the lead-in wire to the detector loop panel in the control cabinet. This splice is frequently broken during construction and traffic signal repair work, but should not pose a problem when proper splicing procedures are used. Finally, a common installation practice in sunbelt jurisdictions is to coat the splice area with hot tar sealant, which can be tracked by vehicles and damage the insulating integrity of the loop wire. This is not recommended. Instead, a preformed splice cover should be used.
Maintenance
Various local agencies and loop installers utilize techniques and shortcuts to expedite analysis of malfunctioning inductive-loop systems. These methods and short cuts can result in poor performance, including adjacent lane detection, motion in the loop wire, and intermittent detection.
A loop detector senses the magnetic field of a vehicle's metal and signals an intersection controller that cars are waiting to proceed. This technology cannot detect pedestrians, motorcyclists and bicyclists.
The sensors must be dug up and can require a lengthy road closure to install or repair. In addition, deteriorating pavement and improper installation can degrade the performance of a loop detector. A new system can help traffic engineers quickly identify failing loops. The Minnesota Department of Transportation's new loop diagnostic software program, detHealth_app, translates the data into four easily understood loop detector health levels. These indicate if the sensor is healthy, tolerable, impaired or nonfunctional. This approach is a significant improvement over current methods.
Design
Inductive loops are one of the most commonly used sensors in traffic signal control. They consist of one or more turns of insulated wire in a slot saw cut in the road surface at the location where vehicles will pass and need to be detected. During the passage of vehicles over the loop, the magnetic field in the roadway is disturbed and this change is sensed by an amplifier housed in the intersection controller cabinet.
You will see these loops at most intersections as large rectangle shaped areas in the pavement right before the stop bar. They are powered by a lead-in cable from the curbside pull box to an electronic unit in the controller cabinet. Vehicles pulling up to the stop bar actuate these loops and notify the controller that there is a car waiting to turn.
Traffic Loop Detectors, and not pedestrians or motorcyclists. They also require that the roadway be dug up and this can impede traffic flow and cause long delays at the intersection. Loops must be installed properly and maintained to provide accurate results. New above ground detection technologies such as thermal cameras and radar can address some of the limitations of traditional loop technology.
