Effect of Speed Monitoring Displays on Entry Ramp Speeds at Rural Freeway Interchanges

The effectiveness of speed monitoring displays (SMDs) in controlling the speed of on-ramp traffic at rural freeway interchanges was evaluated as part of the Midwest States Smart Work Zone Deployment Initiative, a pooled-fund study sponsored by Iowa, Kansas, Missouri, Nebraska, and the Federal Highway Administration. SMDs were deployed along two on-ramps of I-80 eastbound near Lincoln, Nebraska. One SMD was operated on each ramp for a two-week period. The mean, 85th percentile, and standard deviation of vehicle speeds, and the percentage of vehicles complying with the 55-mph speed limit and the 60 and 65-mph speed thresholds were used as measures of effectiveness (MOEs). The SMDs were found to be effective in lowering speeds, increasing the uniformity of speeds, and increasing speed-limit compliance during the first week of their operation. Greater speed reductions and compliance increases were observed for passenger cars than for other vehicles. However, after two weeks of continuous operation, the SMDs began to loose their effectiveness. It is suspected that the compliance reduction is associated with the relatively high percentage of commuting traffic on the ramps.