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The use of design speed as a primary factor in selecting a roadway's horizontal and vertical alignments was initiated in the United States in the 1930s. Since that time, highway design criteria were developed to suggest appropriate horizontal curve radii, superelevation rates and vertical curve elements for new roadways, based on the selection of design speed.
The practice of basing posted speed limits on statistical analysis of individual vehicular speeds observed at a spot on the roadway was initiated at about the same time. This procedure has been followed since the 1930s, and the engineering profession has accepted it as an effective and reasonable procedure. An assumption basic to the procedure is that motorists can decide the appropriate speed at which to travel, and the 85th percentile speed is assumed as a reasonable speed for use as the posted speed limit.
Because of differences in design and operations criteria, there are locations where 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. Consequently, it is not surprising that motorists feel comfortable traveling at speeds greater than the roadway's design speed during good weather conditions; however, when posted speed exceeds design speed, exposure to tort liability, whether or not legitimate, is a concern.
The Texas Department of Transportation (TxDOT) sponsored a study' to document concerns and difficulties about the relationship between design, operating and posted speeds, and to identify methods being used to address these concerns. Several recent studies examined the relationship between design speed and operating speed on rural, two-lane highways. The collection of data on suburban roadways in this study added to the body of knowledge. The four objectives developed to achieve the study's goals were: * Identify current state and national concerns, including liability concerns. * Identify methods being used to address the concerns. m Recommend guidelines for establishing design and posted speeds. * Determine the relationship between design and operating speed on suburban highways using data collected at selected sites.
Three data-collection techniques were used during the TxDOT study: mail-out surveys, personal interviews and field studies.
Mail-Out Surveys
Mail-out surveys were distributed to each TxDOT district...