Since the 1960's the state of Oklahoma has used the turned-down terminal on its W-beam guardrails. In the late 1960's the California Division of Highways and Southwest Research Institute conducted several tests which indicated this turned-down terminal could potentially launch an impacting vehicle and cause it to roll over. A relatively simple method of modifying the turned-down end guardrail terminal has been developed which will eliminate or greatly minimise the probability of a vehicle ramping and rolling over. The modified design uses a 6 in. x 8 in. (150 mm x 200 mm) timber as the first post, a 12 ft-6in. (3.8 m) space where a second 6 in. x 8 in. (150 mm x 200 mm) timber post is placed. Thereafter standard 6 in. x 8 in. posts with 6 in. x 8 in. blockouts are placed on 6 ft-3in. (1.9 m) centres. The first eight posts have 2 in. (50 mm) holes drilled in the 8 in. (200 mm) side. The guardrail is not bolted to the first eight posts. With the bolts removed, the rail will be depressed to the ground when the turned-down terminal piece is impacted by a vehicle. This action minimises or eliminates the undesirable violent vehicle ramp and rollover behaviour. In order to hold the rail at the proper height (29 in. (74. m) before and during vehicle angle impact along the length of need, backup plates are bolted to the first six posts. Successful crash tests as recommended by Transportation Research Circular No. 191 have been conducted to verify this behaviour. (FHWA)
Abstract