RELATIONSHIP BETWEEN DESIGN, OPERATING AND POSTED SPEEDS UNDER HIGH-POSTED SPEED CONDITIONS

The overall objective of this research was to investigate the relationship between design, operating and posted speed on rural two-lane two-way highways in Nebraska with posted speeds of 55, 60, and 65 mph. Crest vertical curvature and horizontal curvature on a sample of these existing highways in Nebraska were investigated to determine if changes in vertical and horizontal alignments affected the operating speeds of motorists using the roadway. Operating speed equations were developed to predict mean, 85th percentile and 95th percentile speeds at crest vertical curves at the point of least available sight distance, horizontal midpoint locations, and locations where motorists are assumed to be driving at their desired speeds. Operating speeds were found to be affected by posted speed, approach tangent grade and average daily traffic volume at crest vertical curve locations and desired speed locations. Operating speeds at the midpoint of horizontal curvature were found to be influenced by the posted speed, deflection angle of the tangents, arc length of the curve, approach tangent grade and average daily traffic volume. Procedures were suggested to help determine: a reasonable design speed to use on a new or reconstructed highway; a reasonable initial posted speed on a new or reconstructed highway; and a reasonable posted speed for an existing highway.