Study Details
Study Title: Evaluation of the Safety Effectiveness of the Conversion of Two-Lane Roadways to Four-Lane Divided Roadways: Bayesian vs. Empirical Bayes
Authors: Ahmed et al.
Publication Date:JAN, 2015
Abstract: This paper utilized various observational before-after analyses to evaluate the safety effectiveness of widening urban and rural two-lane to four-lane divided roadways. These methodologies ranged from simple (naive) before-after, before-after with comparison group, Empirical Bayes (EB), and Bayesian approach. The EB method requires Safety Performance Functions (SPFs) to be calibrated; the simple AADT-based SPF is used widely. In this paper, two sets of Negative Binomial models were calibrated; 'full' SPF model that utilizes various explanatory covariates and 'simple' SPF using AADT only. The preliminary results from the calibrated models indicated that the SPF is pivotal in the EB method; the more accurate the models, the more pragmatic the evaluation of the safety effectiveness of a treatment. The proposed methodology of using the 'Full' SPF in EB method is recommended over the conventional EB observational before-after. In order to obtain more reliable estimates, Bayesian before-after approach was performed. The Bayesian Bivariate Poisson-Lognormal approach provided comparable results and might have several advantages over the EB technique. The results from this paper indicated that the conversion from two-lane roadways to four-lane divided roadways resulted in a notable reduction of more than 63 percent on urban roadways and 45 percent reduction on rural roadways for fatal and injury (F+I) crashes. Conversion to 4-lane divided roadway yielded a higher reduction in total and property damage only crashes in urban areas than in rural areas. Additionally, the safety effects of the conversion appear to be more effective on roadway segments with high AADT in urban areas.
Study Citation: Ahmed, M.M., M. Abdel-Aty, and J. Park. "Evaluation of the Safety Effectiveness of the Conversion of Two-Lane Roadways to Four-Lane Divided Roadways: Bayesian vs. Empirical Bayes". Presented at the 94th Annual Meeting of the Transportation Research Board, Paper No. 15-0830, Washington, D.C., (2015).
CMFs Associated With This Study
Category: Roadway
Countermeasure: Convert 2 lane roadway to 4 lane divided roadway
| CMF | CRF(%) | Quality | Crash Type | Crash Severity | Roadway Type | Area Type |
|---|
| 0.341 | 65.88 |  | All | All | Not specified | Urban |
