Module 2 : Traffic Measurement Procedures
Lecture 08 : Automated Traffic Measurement
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Infrared Sensors The sensors are mounted overhead to view approaching or departing traffic or traffic from a side-looking configuration. Infrared sensors are used for signal control; volume, speed, and class measurement, as well as detecting pedestrians in crosswalks. With infrared sensors, the word detector takes on another meaning, namely the light-sensitive element that converts the reflected or emitted energy into electrical signals. Real-time signal processing is used to analyze the received signals for the presence of a vehicle.
  1. Passive Infrared (PIR)
    Detection of vehicle based on emission or reflection of infrared (electromagnetic radiation of frequency $ 10^{11} - 10^{14} Hz$) radiation from vehicle surface, as compared to ambient levels emitted or reflected from the road surface shown in Fig. 1. The PIR system collected following parameters: Flow volume, Vehicle presence, and detection zone occupancy. Speed with unit with multiple detection zones.

    Figure 1: Emission and reflection of energy by vehicle and road surface. (Source: FHWA vehicle detection manual)
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    Advantages
    1. Relatively long wavelength of light used in PIR systems makes them less susceptible to weather effects.
    Disadvantages
    1. Accuracy of speed information is poor with low resolution sensors. Vehicle length determination is highly problematic for the same reason.