Relationship of design, operating, and posted speeds on horizontal curves of rural two-lane highways in Nebraska

Circular horizontal curves on rural two-lane highways in Nebraska with posted speeds of 55, 60, and 65 mph were investigated to determine the relationship of design and operating and posted speeds in an effort to provide guidelines for consistent roadway design along horizontal alignments. The mean, 85th percentile, and 95th percentile speeds; upper limit of the 16-km/h (10-mph) pace; and percent of vehicles within the pace of free-flowing passenger cars in dry, daytime conditions were analyzed at tangent approach and curve midpoint locations. Highway design guidelines suggest that the posted speed should represent the 85th-percentile speed of the vehicles using the facility and that the roadway alignment should be designed to support the 95th percentile speed. Multiple regression analysis was used to develop prediction equations for the mean, 85th percentile, and 95th percentile speeds at approach and curve midpoint locations. At the midpoint, the deflection angle and length of curve influenced the mean, 85th percentile, and 95th percentile speeds. As the posted speed increased so did the mean speed; as the approach grade increased, the 85th percentile speed decreased; and as the average daily traffic (ADT) increased, the 95th percentile speed decreased. Inferred design speed based on the 2001 AASHTO model does not appear to have an influence on 95th percentile operating speeds in Nebraska. At the approach locations, the 85th percentile and 95th percentile speeds were influenced by posted speed and ADT. The majority of drivers tend not to significantly reduce their speed when traveling from a tangent segment to a horizontal curve (for curves with radii greater than 350 m [1,146 ft]).