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ITS applications rely traffic flow sensors to provide vehicle detection; incident detection; ramp metering data; real-time traffic adaptive signal control; roadway volume and vehicle classification archival and planning data; and data for traveler, commercial, and emergency information services. The success of these ITS applications depends largely on the proper design, installation, and maintenance of sensor components.
Over the past 17 years, sensor manufacturers have developed new, more effective vehicle detection technologies, particularly for over-roadway sensors. During the same period, the manufacturers discontinued some over-roadway and in- roadway models. The new hardware is better able to meet the expectations of the ITS community.
To educate the transportation community on the latest sensor technologies, the Federal Highway Administration (FHWA) recently published a revised and restructured edition of the Traffic Detector Handbook (FHWA-HRT-06-108 and FHWA-HRT-06-139), a two-volume, comprehensive reference on sensors for traffic management on surface streets, arterials, and freeways. Previous editions of the handbook, published in 1985 and 1990, required updating to reflect the evolution, maturation, and state of the practice of sensor hardware and installations.
Antoinette Wilbur, former director of the FHWA Office of Operations Research and Development, writes in the revision's foreword: "The objective of the third edition of the Traffic Detector Handbook is to provide a comprehensive reference document to aid the practicing traffic engineer, planner, or technician in selecting, designing, installing, and maintaining traffic sensors for signalized intersections and freeways." She adds, "The information contained in this handbook is based on the latest research on available treatments and best practices in use by jurisdictions across the United States and elsewhere."
The revised handbook discusses selecting, configuring, installing, operating, and maintaining traffic sensors, along with new applications of sensors to advanced signal control, ramp metering, incident detection, efficient corridor operation, toll collection, collection of travel time and other data, priority vehicle and pedestrian detection, vehicle/driver safety, and other ITS functions. The enhanced descriptions of ITS applications and the other improvements discussed in the revised edition can help departments of transportation (DOTs) ensure long-term savings of public funds.
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Types of Sensors
As defined in the handbook, "A traffic flow sensor is a device that indicates the presence or passage of vehicles and provides data or information that supports traffic management applications such as signal control, freeway mainline and ramp control, incident detection, and gathering of vehicle volume and classification data to meet State and Federal reporting requirements."
In-roadway sensors are embedded in the pavement or the subgrade, or they are taped or otherwise attached to the surface of the roadway. Over-roadway sensors are mounted above the roadway or alongside it.
One of the main types of in- roadway sensors is the inductive- loop detector, which consists of loops of wire embedded into sawcuts in the road pavement. A conductive metal object, such as a vehicle passing over or stopped within the sensor's detection area, decreases the loop's inductance (an electrical property), producing an electrical signal that is transmitted through a curbside junction box (a "pull box") to an electronics unit housed in a controller cabinet. The electronics unit analyzes the signal, interpreting it as the presence or passage of a vehicle, and sends an appropriate call to the controller.
According to the revised handbook, "Today, the inductive-loop detector is, by far, the most widely used sensor in modern traffic control systems." The handbook clarifies the calculations required to...
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