Design Speed,Operating Speed,and Posted Speed Survey

Abstract

At some locations, the posted speed limit, based on 85th percentile operating 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, that is, roadways are designed for near worst-case conditions. When this situation occurs, however, liability concerns can arise although drivers can safely exceed the design speed. A mail-out survey for state, city, and county transportation departments was conducted to document concerns and difficulties the engineering profession is experiencing with the relationships among design speed, operating speed, and posted speed. The factors most frequently considered when selecting design speed are anticipated operating speed and/or posted speed; however, these were not the most common factors selected by respondents. Urban versus rural and functional class were selected more often by the state respondents, whereas cross section was selected more often by the city/county respondents. Most respondents indicated that a ball bank indicator is used to set advisory speeds on horizontal curves. Many respondents expressed the desire to have a new procedure for selecting advisory speeds. Several agencies acknowledged that they have or have had sites with an operating speed greater than the design speed. The most frequent action taken was to install advance warning signs. Few respondents stated that they reduce the posted speed limit to match the design speed. Although few lawsuits involving the posted speed/design speed issue have occurred, liability concerns still exist. Several respondents provided comments that indicated their concern with current definitions and procedures.