Compatibility of design speed, operating speed, and posted speed

Design speed is used in selecting the vertical and horizontal elements for new roadways while speed limits are based on a statistical analysis of individual vehicular speeds. At some locations, the posted speed limit based on an 85th percentile speed exceeds the roadway's design speed. This situation is a result of the fact that criteria used in highway design incorporate a significant factor of safety - i.e., roadways are designed for near worst-case conditions. When posted speed exceeds design speed, however, liability concerns arise even though drivers can safely exceed the design speed. Research conducted in this project clearly indicated that DOT officials are concerned with the potential liability; however, only a few of the respondents to surveys and interviews actually experienced a lawsuit relevant to the design speed-posted speed issue. The respondents indicated that the primary liability concern rests with the current AASHTO definition of design speed. If the definition were changed to reflect its actual meaning, then liability concern would be reduced substantially. During this project, researchers conducted field studies on suburban highways at horizontal curves and limited sight distance crest vertical curves. The field studies found that inferred design speed (for vertical curves) and curve radius (for horizontal curves) are moderately good predictors of the 85th percentile curve speeds. For design speeds less than 90 km/h, the regression equation developed based on the vertical curve field data predicts 85th percentile speeds that are greater than the design speed of the curve. The horizontal curve findings demonstrated that the 85th percentile driver operates at speeds less than design speed on curves with inferred design speeds greater than 70 km/h.