Analysis of Inconsistencies Related to Design Speed, Operating Speed and Speed Limits

Abstract

The objective of this research was to examine the relationship among design speeds, operating speeds and speed limits and address safety and operational concerns regarding the presence of disparities among these speed metrics. Roadway sections were selected throughout Kentucky based on the relationship between design speed and posted speed limit (greater or lower) and on the number of lanes (2 or 4). Speed data and roadway geometry data were collected along these sites to allow for the development of the appropriate models. The general conclusion for 2-lane highways is that the operating speed is different than the design speed indicating that there is no agreement between them. For the 4-lane highways there was an agreement between operating and design speeds indicating the absence of any differences. The relationship between operating speed and posted speed limit showed that for all roadways these two speed metrics were different and the posted speed limit was lower than the 85th operating speeds. The safety analysis showed in general that there were no significant safety consequences from the inconsistencies among the various speeds metrics. A set of recommended guidelines is proposed that aim in alleviating potential inconsistencies among these speed metrics focusing on selecting the design speed based on desired operating speeds to avoid possible inconsistencies that could lead to driver errors.