Factors Influencing Operating Speeds and Safety on Rural and Suburban Roads

Boodlal, Leverson; Donnell, Eric T.; Porter, Richard J.; Garimella, Dileep; Le, Thanh; Croshaw, Kevin; Himes, Scott; Kulis, Philip Nicholas; Wood, Jonathan · 2015 · Unknown

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Summary

This report, produced by the Federal Highway Administration (FHWA), addresses the critical safety issue of speeding on rural and suburban roadways, which are not part of the interstate system. Speeding-related crashes account for nearly one-third of all fatal crashes in the United States, with local streets and collectors exhibiting the highest fatality rates per vehicle mile driven. The primary objective was to identify and evaluate engineering treatments that induce drivers to self-select appropriate operating speeds, thereby reducing speed-related crashes. The study focused on three specific interventions: high-friction surface treatments (HFST) on horizontal curves, optical speed bars (OSB), and the safety effects of lane-width–shoulder-width combinations. The research was conducted in two phases. The first phase involved a comprehensive literature review to identify design features and current practices associated with safer operating speeds. The second phase consisted of field evaluations to determine the effectiveness of specific treatments. HFST was evaluated at four treatment sites and three control sites in West Virginia, analyzing speed, encroachment, and friction data before and after installation. Optical speed bars were implemented and assessed at seven sites in Massachusetts, four in Arizona, and eight in Alabama, testing two different design configurations. Additionally, the safety effects of lane-width–shoulder-width combinations were estimated using crash data from rural two-lane, two-way road segments in Minnesota and Illinois. Statistical analyses included mean operating speed comparisons, speed variance analysis, and models estimating expected crash frequency and severity. The findings revealed mixed results across the evaluated treatments. For high-friction surface treatments, while friction supply clearly increased, there were no consistent differences in operating speeds or vehicle encroachment between the before and after periods. Optical speed bars proved unsuccessful; results yielded inconsistent speed reductions across all test sites, leading to the conclusion that the specific OSB designs tested did not effectively reduce vehicle speeds. In contrast, the analysis of lane-width–shoulder-width combinations provided significant safety insights. The study found that the expected number of total crashes increases as lane width decreases, though distinguishing the performance of 11-foot lanes from 12-foot lanes was difficult. Shoulder width had a main effect of decreasing the expected number of crashes as it increased. Crucially, the interaction between lane and shoulder width showed that shoulder width has the greatest safety effect when lane width is 10 feet, and a greater effect when lane width is 11 feet compared to 12 feet. These results provide specific guidance for roadway design and retrofitting. The lack of speed reduction from HFST and OSB suggests these treatments may not be effective standalone solutions for speed management on curves, despite HFST’s benefit in friction. The strong correlation between shoulder width and crash reduction, particularly on narrower lanes, highlights the importance of cross-sectional geometry in safety planning. The report concludes by offering considerations for future research and providing a catalog of traffic engineering treatments with details on their design, safety effectiveness, and cost, aiding safety professionals in selecting appropriate countermeasures for rural and suburban environments.

Key finding

High-friction surface treatments and optical speed bars were ineffective at reducing operating speeds, whereas increasing shoulder width significantly improved safety by reducing crash frequency, particularly on narrower lanes.

Methodology

field_study

Provenance

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Summary generated by qwen3.6-27b-prismaquant on 2026-06-10; verification: verified.

Topics

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• perceptual countermeasures