Investigating exposure measures and functional forms in urban and suburban intersection safety performance functions...

Area of Science:

• Traffic Engineering and Safety
• Transportation Research
• Statistical Modeling in Transportation

Background:

• Accurate prediction of intersection crashes is vital for effective safety interventions.
• Traditional Safety Performance Functions (SPFs) often struggle with complex relationships between exposure and crash types.
• The choice of exposure measure and functional form significantly impacts SPF reliability.

Purpose of the Study:

• To propose and evaluate the Generalized Negative Binomial-P (GNB-P) model for intersection crash prediction.
• To identify the most reliable exposure measures (major road AADT, minor road AADT, total AADT) for different crash types.
• To determine the most appropriate functional forms (Power, Hoerl 1, Hoerl 2) for Negative Binomial (NB) models.

Main Methods:

• Development and application of the GNB-P model to analyze crash data for stop-controlled and signalized intersections.
• Estimation of three SPF functional forms using various exposure measures for different crash types.
• Modeling over-dispersion using exposure measures to account for crash data variability.

Main Results:

• The mean-variance structure of NB models and over-dispersion vary significantly by crash data and intersection type.
• Minor road AADT positively correlates with over-dispersion for Same-Direction Crashes (SDC), Intersecting-Direction Crashes (IDC), and Single-Vehicle Crashes (SVC).
• Specific functional forms and exposure measures demonstrated superior performance for different crash types (e.g., Power function for Opposite-Direction Crashes (ODC), Hoerl function 2 for SVC, Hoerl function 1 for SDC and IDC).

Conclusions:

• The GNB-P model provides a more robust approach to modeling intersection crash counts and their relationship with exposure measures.
• Estimating over-dispersion using exposure measures leads to more reliable SPF predictions.
• The study offers guidance on selecting optimal exposure measures and functional forms for intersection SPFs based on crash type.