Study Details
Study Title: Estimating the safety effects of lane widths on urban streets in Nebraska using the propensity scores-potential outcomes framework
Authors: Wood et al.
Publication Date:SEP, 2015
Abstract: A sufficient understanding of the safety impact of lane widths in urban areas is necessary to produce geometric designs that optimize safety performance for all users. The overarching trend found in the research literature is that as lane widths narrow, crash frequency increases. However, this trend is inconsistent and is the result of multiple cross-sectional studies that have issues related to lack of control for potential confounding variables, unobserved heterogeneity or omitted variable bias, or endogeneity among independent variables, among others. Using ten years of mid-block crash data on urban arterials and collectors from four cities in Nebraska, crash modification factors (CMFs) were estimated for various lane widths and crash types. These CMFs were developed using the propensity scores-potential outcomes methodology. This method reduces many of the issues associated with cross-sectional regression models when estimating the safety effects of infrastructure-related design features. Generalized boosting, a non-parametric modeling technique, was used to estimate the propensity scores. Matching was performed using both Nearest Neighbor and Mahalanobis matching techniques. CMF estimation was done using mixed-effects negative binomial or Poisson regression with the matched data. Lane widths included in the analysis included 9 ft, 10 ft, 11 ft, and 12 ft. Some of the estimated CMFs were point estimates while others were functions of traffic volume (i.e., the CMF changed depending on the traffic volume). Roadways with 10 ft travel lanes were found to experience the highest crash frequency relative to other lane widths. Meanwhile, roads with 9 ft travel lanes were found to experience the lowest relative crash frequency. While this may be due to increased driver caution when traveling on narrow lanes, it is possible that unobserved factors influenced this result. CMFs for target crash types (sideswipe same-direction and sideswipe opposite-direction) were consistent with the values currently used in the Highway Safety Manual (HSM).
Study Citation: Wood, J.S., J.P. Gooch, and E.T. Donnell. "Estimating the safety effects of lane widths on urban streets in Nebraska using the propensity scores-potential outcomes framework". Accident Analysis and Prevention, Vol. 82, (2015) pp. 180-191.
Study Report: Download the Study Report Document
CMFs Associated With This Study
Category: Roadway
Countermeasure: Decrease lane width from 11 feet to 10 feet
| CMF | CRF(%) | Quality | Crash Type | Crash Severity | Roadway Type | Area Type |
|---|
| 1.27 | -27 |  | All | All | All | Urban |
 | | | All | K,A,B,C | All | Urban |
Countermeasure: Decrease lane width from 11 feet to 9 feet
| CMF | CRF(%) | Quality | Crash Type | Crash Severity | Roadway Type | Area Type |
|---|
| 0.53 | 47 |  | All | All | All | Urban |
Countermeasure: Reduce lane width from 10 ft to 9 ft
| CMF | CRF(%) | Quality | Crash Type | Crash Severity | Roadway Type | Area Type |
|---|
| 0.57 | 43 | | All | All | All | Urban |
Countermeasure: Reduce lane width from 12 ft to 10 ft
| CMF | CRF(%) | Quality | Crash Type | Crash Severity | Roadway Type | Area Type |
|---|
| 1.28 | -28 | | All | All | All | Urban |
Countermeasure: Reduce lane width from 12 ft to 11 ft
| CMF | CRF(%) | Quality | Crash Type | Crash Severity | Roadway Type | Area Type |
|---|
| | | All | All | All | Urban |
| | | All | K,A,B,C | All | Urban |
| | | Sideswipe | All | All | Urban |
| | | Rear end | All | All | Urban |
Countermeasure: Reduce lane width from 12 ft to 9 ft
| CMF | CRF(%) | Quality | Crash Type | Crash Severity | Roadway Type | Area Type |
|---|
| | | All | All | All | Urban |
