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Federal Highway Administration Research and Technology
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Publication Number: FHWA-HRT-06-139
Date: October 2006

Traffic Detector Handbook:Third Edition—Volume II


Sensor installation may appear to be a relatively easy task once the operation of candidate sensor technologies is understood, the suitability of a particular technology to an application is reviewed, and the design, specifications, and plans are developed. However, the mechanical operations associated with sensor installation often present challenges. For example, inductive-loop detector installation requires approved procedures for cutting a slot in the pavement, laying turns of wire in the slot, covering them with sealant, removing excess sealant, splicing the wire to the cable, and connecting the cable to the electronics unit in the controller cabinet. For over-roadway sensors, analogous tasks and related issues appear, such as installation of mounting structures, power and data cables, sensor alignment, and calibration verification.

In reality, the seemingly uncomplicated activity of sensor installation is composed of several critical processes. Improper or sloppy installation causes many of the sensor failures and signal malfunctions that are observed. For example, consider that the detection element represents about 4 to 10 percent of the cost of implementing intersection-traffic control, yet accounts for a significant portion of maintenance dollars. Moreover, dependence on properly functioning sensors increases as traffic congestion and delay become a more urgent problem and traffic-control equipment becomes more sophisticated.

This chapter addresses the needs of project engineers, contractors, inspectors, field crew supervisors, and traffic technicians by reviewing the proper techniques for installing traffic-flow sensors; and it discusses preinstallation activities that apply to all types of sensors. To underscore the criticality of the installation process, the magnitude and causes of inductive-loop detector failures are addressed next. Inductive-loop detector installation procedures for existing roadways are then described, followed by a discussion of loop-installation procedures for new and repaved roadways. Installation of magnetometers, magnetic detectors, and over-roadway sensors (e.g., video image processors, microwave and laser radars, passive infrared sensors, ultrasonic sensors, passive acoustic sensors, and sensor combinations) are discussed in sequence in the latter sections of the chapter.


The design of the sensor system involves a number of decisions that are made previous to installation. Examples of these decisions for in-roadway sensors utilized for signal control were discussed in Chapter 4. Included there are specifications for the type and configuration of the sensor hardware necessary to meet previously identified operational and data requirements, and a preliminary plan showing the sensor layout at the selected location.

Preinstallation activities should include a thorough review of the design documents, preparation of scale drawings, and field visits to the location. Upon completion of these activities, the engineer is ready to hand the job over to the installation crew foreman or the contractor, who must then develop manpower estimates and material and equipment requirements. All information should be clearly defined and complete so that installation can proceed in an orderly and effective manner. The implication of these activities is discussed further in the following paragraphs.


A scale drawing of the location showing the geometry of the roadway and the location of the in-roadway or over-roadway sensor components in relation to the pavement markings or mounting structure is needed. The drawing should depict the location and content of conduit, manholes, power sources, pavement materials, and electrical equipment that interface with the installation. Because it provides fundamental guidance for the installation crew and becomes part of the procurement package needed for a contractor installation, the accuracy of this drawing directly affects sensor installation.

The completed drawing should be reviewed with the design engineer to insure that the sensors are located as specified in the configuration design and in areas free of underground or under pavement hardware or overhead obstructions (in the case of over-roadway sensors) that could interfere with proper operation.


Field visits to the installation site are required before and after the scale drawing is made. The field visit made prior to drawing preparation is for inventorying the existing conditions and identifying potential problem areas. These include pavement joints, which may affect the design of an inductive-loop detector system, and lines of sight and identification or lack of support structures, which affect installation of over-roadway sensors. Other factors to be noted during the site visit are listed in the “Video Image Processors” section under “Site Surveys.”

As a result of this visit, the project engineer should determine the method for burying cables, installing in-roadway and over-roadway sensors, and the types of equipment needed for the installation strategy. Methods for traffic routing and control during installation should also be determined, including the position and quantity of barricades or cones that may be needed for traffic control. Finally, the permits and licenses required for installation should be identified.

The information obtained from the initial visit should be incorporated into the plan drawings. After the drawings are completed, a second field visit is made to verify their accuracy.


The size and skills required of the installation crew should be estimated prior to installation. Plan drawings should provide sufficient information to determine the scope of the installation tasks, the amount of time required, and the size of the crew necessary for the installation. Depending upon the size of the crew, many of the tasks may be performed simultaneously, thereby reducing the time traffic is disrupted.


The sensor types and layout dictate the equipment required for the installation. Table 5-1 lists equipment commonly needed for a typical inductive-loop detector installation. Barricades, signs, cones, safety vests, and other devices are also necessary to control traffic safely during the installation process.

In addition, magnetometer installation requires a drill and bit to bore the vertical hole in the roadway for the sensor probe. Magnetic detector installation usually requires a backhoe, a horizontal boring machine, a level, and a tamping machine.

Over-roadway sensor installation usually requires a bucket truck.

Table 5-1. Equipment for typical inductive-loop detector installation.
ItemDescription and use
Power saw
Self-propelled 18–65 horsepower (hp) saw equipped with 1 / 4 to 3 / 8 inch (6.4 to 9.5 mm) thick blade (abrasive or diamond), water valve, depth gauge, and horizontal guide
Water supply
Used with diamond blade for cooling and cleaning out of sawcuts
Bores holes through concrete curb
Air compressor
Used with jackhammer to clean and dry sawed cuts
Chisel and hammer
Removes sharp edges at corners of sawcuts
Blunt tool
For example, a wooden stirring stick for seating wire in saw slot
Provides uniform twists of the lead-in wires
Template or straight edge
Marks outlines of loops on pavement
Trenching machine
Used to bury cable in dirt
Megohmmeter and volt/ohm meter
Used to inspect and test wire continuity and resistance
Loop analyzer
Tests continuity and inductance of the wire loop
Soldering iron
Either a butane torch with a soldering tip or an electric soldering iron for making solder connections at the splice
Measuring tape
100 ft (30 m) minimum-length tape for making exact measurements for loop placement


As with manpower and equipment, the type and amount of installation materials should be determined. Material quantities should be ample to avoid any interruptions of work. A typical material list for inductive-loop detector installations is provided in Table 5-2. Other types of power and data cables and suitable mounting brackets are required when installing over-roadway sensors.

Table 5-2. List of materials for typical inductive-loop detector installation.
Detector wire
Forms the loop and lead-in wires
Lead-in cable
Spliced to the loop lead-in wire in the pull box, it connects the lead-in wires to the electronics unit in the controller cabinet
Pull box
Contains the connection (splice) between the lead-in wire and lead-in cable and provides access for maintenance
Seals the loop wire in the sawcut and protects the loop from environmental damage
Cement, sand, or talc
Dusted on sawcut after sealant is applied to prevent tracking
Holds the pull box in place
Surge voltage protector
Provides protection for the electronics in the controller cabinet
Needed to make splices
Splice kits (or equivalent)
Environmentally seals the splices
Spray paint or chalk and line
Outlines loop dimensions prior to sawcutting

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