A Multi-Site Examination for the Impact of Changes in Posted Speed Limit on Traffic Safety

Abstract

Introduction

Despite numerous studies reporting the negative impacts of increased speeds on traffic safety, many states have raised their posted speed limits. In response, the AAA Foundation for Traffic Safety (AAA Foundation) initiated a multiphase study in 2018 to investigate the effects of posted speed limit changes. The first phase entailed gathering feedback from traffic engineers on how posted speed limits are set and what factors they consider in changing posted speed limits. The second phase involved a collaborative effort with the Insurance Institute for Highway Safety and Humanetics Innovative Solutions to examine how vehicle crashworthiness and occupant protection degrade as impact speed increases. The final phase comprised before-and-after assessments of crash and speed profile data collected from sites where posted speed limits were raised or lowered, and examination of any differences between sites.

Key Findings

In general, the results varied across all sites examined. Inconsistent results were found in the sites within the same state and within the same road functional class. Even within the site, the results were sometimes mixed across different measures. For example, among sites where posted speed limits were raised, crash rates increased at some sites but decreased at others. Similarly, the direction of changes in crash rates varied among sites in the same state where speed limits were lowered. The only consistent results were regarding the likelihood of speed limit violations: after raising posted speed limits, vehicles were less likely to exceed the limits, and after lowering they were more likely to exceed the speed limits. With regard to travel times and traffic volumes, no consistent associations with changes of posted speed limit were found.

The reasons for mixed results may include the differences in characteristics of each roadway (e.g., road geometric designs, environmental factors), driving behaviors, and traffic safety culture, among many other factors. Additionally, differences in data quality, availability, and collecting system can lead to inconsistent results.

Despite the varying results in the current study, the AAA Foundation’s past studies, along with other literature, suggest that practitioners should apply integrated or holistic approaches when setting or changing posted speed limits. Such approaches should take into consideration a variety of factors (beyond the 85th percentile operating speeds) and contexts to guide the selection of safe, credible, and enforceable speed limits for various facility types. Furthermore, a rigorous assessment using multiple measures is critical after implementing a new speed limit, since the results may vary by measure and site as in the current analysis.

Methodology

This study included 12 roadway segments where the posted speed limits were changed—six raised and six lowered—between 2014 and 2018. Sites were in MD, ME, TX, OR, and CA and included a variety of road functional classes (e.g., freeways, arterials, collectors) to compare and contrast results across different geographical characteristics.

This study used data fusion techniques to generate working datasets for in-depth analyses. Comprehensive attributes related to each reported crash were obtained by merging crash, speed, travel time, and traffic volume data. Using the fused datasets, descriptive analyses were conducted and logistic regression models were constructed reflecting safety, driver behaviors, and traffic operations.

Thanks to Dewberry Engineers, Inc. for their assistance with data fusion and to the Maine DOT for providing some of the data.