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

Area of Science:

• Transportation Engineering
• Traffic Safety Analysis
• Statistical Modeling

Background:

• Safety Performance Functions (SPFs) in the Highway Safety Manual (HSM) model crash frequency using roadway features.
• Traffic volume, measured by average annual daily traffic, is a key predictor in SPFs, typically modeled with a logarithmic transformation.
• Existing models assume constant elasticity between traffic volume and crash frequency, potentially limiting insights into varying safety impacts.

Purpose of the Study:

• To propose and evaluate an alternative functional form for traffic volume in SPFs.
• To allow for varying elasticities between traffic volume and expected crash frequency across different traffic volume ranges.
• To improve the fit and predictive performance of SPFs for roadway safety.

Main Methods:

• Developed a novel functional form for the traffic volume variable in SPFs.
• Applied the proposed functional form to SPFs for two-lane rural roadways in Pennsylvania.
• Compared the performance of the proposed model against traditional and Hoerl functional forms.

Main Results:

• The proposed functional form demonstrated an improved fit compared to traditional and Hoerl forms.
• The new model showed enhanced predictive performance for crash frequency.
• The functional form allows for natural breakpoints, reflecting varying effects of road features at different traffic volumes.

Conclusions:

• The proposed functional form offers a more accurate representation of the relationship between traffic volume and crash frequency.
• This approach may provide better insights into safety performance across diverse traffic conditions.
• Consideration of this functional form is recommended for future SPF development in roadway safety.