Advisory Speed Safety Study

The use of curve warning advisory speed signs at sharp horizontal curve locations is standard throughout the United States. The safety benefits of advisory speed signs are assumed by the engineering community, but there is really no clear research demonstrating the safety evaluation of these signs and their relationship to crashes or changes to safe driver performance at these locations. The criteria for placement and the methods for determining where advisory speed signs should be placed vary between jurisdictions and often within a jurisdiction. Recently, researchers at Oregon State University completed an advisory speed evaluation of a large random sample…

The use of curve warning advisory speed signs at sharp horizontal curve locations is standard throughout the United States.  The safety benefits of advisory speed signs are assumed by the engineering community, but there is really no clear research demonstrating the safety evaluation of these signs and their relationship to crashes or changes to safe driver performance at these locations.  The criteria for placement and the methods for determining where advisory speed signs should be placed vary between jurisdictions and often within a jurisdiction.  Recently, researchers at Oregon State University completed an advisory speed evaluation of a large random sample of 210 state highway horizontal curve locations on rural highways in Oregon.  The purpose of this previous study was to determine the level of compliance with current advisory speed standards and the implications of changing posting criteria to match new and future MUTCD signing guidelines.  During this recently completed effort, the research team determined that there are many current advisory speed signs that do not adhere to Oregon posting criteria. An additional question generated by the initial research effort was if there was a history of safety problems at locations that are not currently in compliance with posting criteria.  This question was beyond the scope of the initial study but has significant financial implications to the State of Oregon. This proposed research, therefore, seeks to evaluate the safety record for the randomly sampled curve locations where possible and contrast this finding with sign compliance in an effort to determine the safety implications of various advisory speed signage treatments.

This proposed research effort seeks to evaluate the currently posted advisory speeds at 210 randomly selected rural highway locations in Oregon.  The historic crash record at these locations will be contrasted with properly posted advisory signs, locations where advisory signs are posted but not in direct correlation with Oregon standards, and locations where advisory speed signs are warranted but not posted.  In addition, a sub-set of sites will be targeted for detailed operating speed assessment to determine how the posted advisory speeds correspond with driver selected operating speeds.  This sub-set of sites will include a variety of companion horizontal curve radii so that the direct influence of road geometry can be included in the assessment.

The next edition of the MUTCD will recommend a modified threshold for advisory speeds that varies significantly from the current Oregon threshold.  Decision makers in Oregon must determine if they want to adopt new MUTCD thresholds and invest in the placement of new advisory speed signs based on these modified thresholds.  In the event current Oregon thresholds are retained, the recent advisory speed study demonstrated that many of the current advisory speed signs do not meet Oregon criteria.  The decision to invest in replacing these signs must also be considered.  This research will aid Oregon decision makers by determining specific safety implications of current posted advisory speeds and the implications of either modifying the posting threshold or enforcing the current threshold.  This proposed research will also help identify priorities for sign upgrades by determining the safety implications of the various compliance levels.