Study Details

Study Title: Context-based Crash Modification Factors for Medians on Rural Four-Lane Roadways: A Bayesian Approach

Authors: Li et al.

Publication Date: JAN, 2021

Abstract: Rural four-lane roadways provide important transportation accessibility and mobility to populations in rural areas. Practitioners are often challenged to determine cross-section types when both benefits and costs need to be considered. The Crash Modification Factors (CMFs) are often developed to evaluate safety effectiveness of alternative designs. However, safety effectiveness could vary significantly across contexts. Thus, the study aims to estimate CMFs for alternative cross-sections of rural four-lane roadways under different contexts characterized by traffic volume, truck percentage, and access point density. Using Georgia state-wide crash data, this study developed Safety Performance Functions (SPFs) to predict crash frequencies for different contexts. Considering linearity and independence assumptions of traditional negative binomial SPFs, this study adopts the Bayesian generalized negative binomial (BGNB) modeling approach to relax those assumptions and only follows the Bayes rule to form SPFs for CMF estimation. This study focuses on four typical cross-sections including 1) non-traversable medians; 2) two-way-left-turn lanes; 3) 4-ft flush medians; and 4) undivided roadways with double-yellow lines (the base cross-section). The results show that CMFs vary significantly across different contexts. Compared with base cross-section design, safety benefits of other three designs can be either positive or negative under different traffic or road conditions. For example, 4-ft flush medians are found to have positive safety benefits (CMF 1) under greater average daily traffic volumes (e.g., >=15,000). The findings offer practitioners insights that cross-section designs may need to be varied for different contexts.

Study Citation: Li, X., J. Liu, C. Yang, T. Barnett. "Context-based Crash Modification Factors for Medians on Rural Four-Lane Roadways: A Bayesian Approach". Presented at the 100th Annual Meeting of the Transportation Research Board, Paper No. 21-00190, Washington, D.C., (2021).

Study Report: Download the Study Report Document


CMFs Associated With This Study

Category: Roadside

Countermeasure: Provide non-traversable median

CMF CRF(%) Quality Crash Type Crash Severity Roadway Type Area Type
CMF Equation CRF Equation 2 Stars All All Principal Arterial Other Rural

Category:Roadway

Countermeasure: Crash modification factor for median width of X ft

CMF CRF(%) Quality Crash Type Crash Severity Roadway Type Area Type
CMF Equation CRF Equation 2 Stars All All Principal Arterial Other Rural

Countermeasure: Install TWLTL (two-way left turn lane) on four lane road

CMF CRF(%) Quality Crash Type Crash Severity Roadway Type Area Type
CMF Equation CRF Equation 2 Stars All All Principal Arterial Other Rural