Investigating the safety effects of road width on urban collector roadways

• Studying the safety implications of oversized urban collector roads.
• Evaluation conducted using cross-sectional Negative Binomial Safety Performance Functions.
• Standard-sized and oversized roads with midblock change were less safe than roads with uniform width.
• Converting oversized roads to standard size improved safety only for the roads with 4000 ADT or more.
• The maximum crash reduction was estimated to be as high as 28.9%.

Collector roads are designed to provide a balance between traffic safety, mobility, and land access; however, the literature on the effects of collector lane width on safety is generally inconsistent. This study determines the safety implications of oversized collectors by analyzing collisions statistics, traffic-survey data, and roadway-inventory data from the City of Edmonton. The paper adopts a cross-sectional evaluation methodology by developing negative binomial (NB) safety performance functions (SPFs) for total collisions. Based on the NB SPFs, it was found that segment length, traffic volume, access-point density and midblock change was statistically significant and positively related to collisions, while the width was negatively related to collisions and statistically significant. The interaction term of volume and lane width was found to be positively related to collisions. This suggests that the collision rate for oversized collector roads is higher than that for standard-sized roads. It was also found that both standard-sized and oversized roads, with some form of midblock roadway width change, were less safe than those with uniform width. Converting oversized roads to standard size was found to improve safety only for the roads with an average daily traffic (ADT) volume higher than 4000. The maximum crash reduction was estimated to be as high as 28.9%. This study is timely given the contradicting evidence that exists in current literature regarding the relationship between safety and oversized collector roads. For the present data set, this paper provides guidelines on when to convert oversized collector roads to standard-sized collector roads, to improve safety.