Modern photoelectric sensors usually consist of a light emitting diode (LED) source and a phototransistor receiver with associated lenses and electronic signal-processing circuits. The LEDs are operated in the infrared wavelengths for efficiency, and are pulse modulated to overcome the adverse effects of ambient light, slight misalignment, and partial obstruction of the beam. These devices are packaged in small, moisture-resistant housings and are readily available, commercially. Photoelectric sensors mounted above, beside, or on the roadway surface can detect the presence of vehicle bodies and tires. A pair of sensors produces the information needed for calculating vehicle speed, direction of travel, overall vehicle length, and axle spacing. An array of three sensors produces information for determining single or dual tires and the lateral position of the tires in the traffic lane. The three-sensor array also makes it possible to estimate the size of the tire-pavement contact patch. Attempts to correlate these measurements with wheel loads and gross vehicle weights have not yielded satisfactory results. However, for many traffic survey applications, reflex-type infrared sensors offer an economical and practical alternative to other types of vehicle and tire sensors, especially for short periods of time. They are commercially available, relatively inexpensive, durable, produce output signals compatible with most traffic recording devices, and can be installed without cutting the pavement surface.
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