Safety Evaluation of Curve Warning Speed Signs

Abstract

Sponsoring Agency Code Supplementary Notes AbstractThis report presents a review of a research effort to evaluate the safety implications of advisory speeds at horizontal curve locations on Oregon rural two-lane highways. The primary goals of this research effort were to characterize driving operations at rural two-lane highway curve locations where advisory speed signs were present, and to determine to what extent these signs play a role in enhancing safety. Placement of advisory speed signs at horizontal curve locations in the State of Oregon is a practice aided by unique and specific state-level policies and, as such, may vary from nationally accepted procedures.Speed data was collected at 16 sites and compliance with advisory speed signs determined. An evaluation of site crash data and how advisory speed relates to historic crash information is included along with a statistical model that identifies critical variables that are associated with the posted speed and how they ultimately relate to the expected crash frequency. The research team developed a statistically based advisory speed model that assesses predicted crash outcomes based on a combination of geometric design, operations, and signage. The resulting advisory speed models were then contrasted to the expected advisory speeds based on the Oregon Policy as well as the 2009 MUTCD thresholds. They determined that the safety-based model actually predicted advisory speeds that are not as conservative as those recommended using the ballbank thresholds in the 2009 MUTCD or those identified using the thresholds Oregon has been using.

How Far from Optimal are Current Advisory Speeds?

“…This shortcoming appeared to be linked to the ball bank and accelerometer approaches. Bonneson et al ultimately recommended the use of the design speed equation approach, which yields The authors of this paper have recently proposed a new crash modification function to account for advisory speeds in their recently completed effort for the Oregon Department of Transportation (1). The authors coined this crash modification function the "advisory speed crash factor" (ASCF).…”

“…The result is said to balance a minimum level of multicollinearity with the meaningfulness of the predictors from the engineering standpoint. Further details on the scope of application and statistical modeling can be found elsewhere (1,2,15).…”

“…The Oregon Department of Transportation has recently adopted the thresholds suggested by the latest edition of MUTCD (6). Oregon previously used more conservative thresholds (1).…”