Abstract: An automatic traffic monitoring system for enforcing traffic laws and regulations and for general purpose traffic monitoring includes a novel passive road sensor that accurately detects the kinematics of moving vehicles. A passive road sensor includes a detector protected in an enclosure, which is embedded in a road opening, is in a continuous listening mode. When the wheels of a passing vehicle come in contact with either the road opening, the enclosure, or both, the resulting mechanical impact generates a disturbance that triggers the detector. A processor unit of the automatic traffic monitoring system records the signal sensed by the detector and analyzes its temporal characteristics to determine the precise time of impact.