GUARDRAIL ENCROACHMENT MONITOR--GEM. SUMMARY REPORT

An experimental instrumentation system for monitoring long lengths of traffic railings to detect encroaching vehicles and measuring impact conditions has been developed and subjected to preliminary evaluation. The system makes use of the traffic railing as part of a transmission line carrying two radiofrequency waves. The radiofrequency energy reflected from vehicles entering the electromagnetic field about the line is detected and used to determine vehicular speed, direction, angle, location, path and size. A solid-state, digital data acquisition system is triggered by a detected encroachment to record pertinent data. Capability to record data on up to 50 encroachments is available. Computer data processing generates plots of vehicle path and tabulates data on time, speed, angle, location of encroachment, vehicle size and nature of encroachment. The system is packaged to be suited for unattended roadside installation and will operate continuously from a self-contained 12 V battery for periods of up to 10 days without servicing. Tests indicate generally satisfactory performance, speed measurement accuracy of plus or minus 0.5 mph (plus or minus 0.8 km/hr), angle measurement accuracy of plus or minus 1 deg, location accuracy of plus or minus 10 ft (plus or minus 3m), parallel range of more than 1300 ft (396m), and perpendicular range of 3 ft (lm) or more. Long term evaluation in a roadside installation is needed to further establish the reliability and limitations. (FHWA)