The Highway Safety Manual (HSM) currently provides safety performance functions (SPFs) for several types of roadways and intersections. Highway agencies can calibrate the SPFs to local conditions for use in predicting expected safety outcomes for given roadway designs and safety features. These SPFs, and others developed in-house by highway agencies, are a function of annual average daily traffic (AADT), since the likelihood of crashes on a roadway is directly related to the number of vehicles using it.
Speed, too, is generally understood to be an important factor in roadway safety performance. The severity of the crash is particularly sensitive to vehicle speeds, since the crash energy increases by the square of the vehicle velocity. Speed may also affect the probability of crash occurrence, although this relationship is less well-understood. Despite the importance of speed on safety, predictive models generally do not yet include speed measures.
Research is needed to understand which speed measures (e.g., average speed, 85th percentile speed, speed variance, speed limit violation rate, etc.), in conjunction with geometrics, cross section, and context, are the best predictors of crash frequency and severity.
The objective of this research is to develop a predictive methodology for rural two-lane, two-way highways, for consideration by the AASHTO HSM Steering Committee, that incorporates speed measures (or surrogates for speed measures) to estimate crash frequency and severity.
Research is in progress on Phase II. The Phase I interim report is available on request to NCHRP.