Determining Level of Service on Freeways and Multilane Highways with Higher Speeds

The Highway Capacity Manual provides a means for evaluating level of service on freeways, highways, and urban streets. Presently, the Highway Capacity Manual methodology is not able to evaluate level of service on freeways with free-flow speeds greater than 75 mph or multilane highways with free-flow speeds greater than 60 mph, and many states now have facilities with posted speed limits that exceed these free-flow speeds. Recent research, conducted by the Texas A&M Transportation Institute (TTI), developed speed prediction equations for uninterrupted flow facilities with higher posted speed limits. From these equations, this paper develops procedures for calculating free-flow speed on facilities with higher posted speed limits and develops methods for estimating level of service on freeways with free-flow speeds up to 85 mph and multi lane highways with free-flow speeds up to 80 mph. For freeways, the recommendations call for utilizing the TTI speed prediction equations to calculate free-flow speed and using the calculated free-flow speeds to determine level of service from speed–flow curves developed within this paper. For multilane highways, recommendations call for utilizing the TTI speed prediction equations to calculate base free-flow speed, which is part of the equation for calculating free-flow speed, and then using the calculated free-flow speed to determine level of service from speed–flow curves developed within this paper. To aid practitioners, step-by-step procedures are provided for applying the TTI speed prediction equations within the Highway Capacity Manual methodology.