Estimating safety performance functions for two-lane rural roads using an alternative functional form for traffic volume

• New SPF functional form proposed that allows traffic volume to have different elasticities in ranges. Proposed for continuous relationship between predicted crash frequency and volume. models shown better fit observed data traditional other forms. Statistical of expected are referred as Safety Performance Functions (SPFs) the first edition American Association State Highway Transportation Officials’ Manual (HSM). The SPFs HSM specify annual frequencies a function various roadway roadside features, with most important predictor variable being volume, which serves measure vehicle exposure crashes. Traffic volumes typically measured using average daily incorporated into natural logarithm transformation. This specification suggests increases non-linearly constant elasticity over range values. While researchers concur is non-linear, further exploration this may offer additional insights concerning association safety performance exposure. paper proposes an alternative within ranges, while preserving same general non-linear existing SPFs. Although forms—like Hoerl function—have been literature, model breakpoints or geometric features might varying effects on low- high-volume roads. was applied developed two-lane rural roadways Pennsylvania. Comparisons forms suggest offers improved predictive performance, thus be considered development future