Study Details
Study Title: Full Bayesian evaluation of the safety effects of reducing the posted speed limit in urban residential areas
Authors: Islam and El-Basyouny
Publication Date:JUL, 2015
Abstract: Full Bayesian (FB) before-after evaluation is a newer approach than the empirical Bayesian (EB) evaluation in traffic safety research. While a number of earlier studies have conducted univariate and multivariate FB before-after safety evaluations and compared the results with the EB method, often contradictory conclusions have been drawn. To this end, the objectives of the current study were to (i) perform a before-after safety evaluation using both the univariate and multivariate FB methods in order to enhance our understanding of these methodologies, (ii) perform the EB evaluation and compare the results with those of the FB methods and (iii) apply the FB and EB methods to evaluate the safety effects of reducing the urban residential posted speed limit (PSL) for policy recommendation. In addition to three years of crash data for both the before and after periods, traffic volume, road geometry and other relevant data for both the treated and reference sites were collected and used. According to the model goodness-of-fit criteria, the current study found that the multivariate FB model for crash severities outperformed the univariate FB models. Moreover, in terms of statistical significance of the safety effects, the EB and FB methods led to opposite conclusions when the safety effects were relatively small with high standard deviation. Therefore, caution should be taken in drawing conclusions from the EB method. Based on the FB method, the PSL reduction was found effective in reducing crashes of all severities and thus is recommended for improving safety on urban residential collector roads.
Study Citation: Islam, M.T., K. El-Basyouny. "Full Bayesian evaluation of the safety effects of reducing the posted speed limit in urban residential areas". Accident Analysis and Prevention, Vol. 80, (2015) pp. 18-25.
CMFs Associated With This Study
Category: Speed management
Countermeasure: Lower posted speed limit from 50 kph to 40 kph
| CMF | CRF(%) | Quality | Crash Type | Crash Severity | Roadway Type | Area Type |
|---|---|---|---|---|---|---|
| 0.74 | 26 |  | All | All | Major Collector | Urban |
| 0.501 | 49.9 |  | All | K,A,B,C | Major Collector | Urban |
| 0.821 | 17.9 | | All | O | Major Collector | Urban |
