Speed limit effectiveness in short-term rural interstate work zones

Average speed and speed variance are two characteristics of traffic flow that affect accident rates and severity near work zones. Operationally, work zones account for approximately 24% of the non-recurring delay. A policy for work zone speed limits has to delicately balance the safety and the efficiency impacts. If speeds are set too low, then avoidable congestion and speed violations might result. If speeds are set too high then, again, safety may be compromised. In this paper, results from field studies conducted on three Interstate 70 maintenance short-term work zones in rural Missouri are presented for three different speed limit scenarios: no posted speed limit reduction, 10 mph posted speed limit reduction, and 20 mph posted speed limit reduction. The 85th percentile speeds and standard deviation of speed were found to be 81 and 10 mph; 62 and 8 mph; and 48 and 6 mph, respectively for the three scenarios. The differences in 85th percentile speed and standard deviation of speed among all three scenarios were statistically significant. The per cent of drivers who exceed speed limit by over 10 mph were 15·4, 4·8, and 0·9%, respectively. Thus a reduction in posted speed limit was effective in reducing prevailing speeds and speed variances in short-term work zones in rural Missouri.