Public Roads: A Journal of Highway Research, Vol. 29, No. 9

Blosser, N.B.; Boswell, H.H.; Brink, R.H.; Chesson, E., Jr; Cope, E.M.; Coppage, E.J., Jr.; Dimmick, T.B.; Duzan, H.C.; Farrell, F.B.; Flynt, R.A.; Grieb, W.E.; Gronberg, G.D.; Halstead, W.J.; Hamner, L.B.; Hopkins, R.C.; Kilpatrick, M.J.; Levin, D.R.; Lewis, R.S.; Liston, L.L.; Mertz, W.L.; Moore, R.W.; Munse, W.H.; O'Flaherty, Daniel; Peek, R.A.; Petring, F.W.; Petroff, B.B.; Prisk, C.W.; Rex, H.M.; Saal, C.C.; St. Clair, G.P.; Samson, Elizabeth; Skelton, R.R.; Solomon, David; Stern, E.L.; Taragin, Asriel; Timms, A.G.; Todd, T.R.; Walker, W.P.; Werner, George; Winfrey, Robley · 1957 · ROSA P / United States. Government Printing Office

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Summary

This 1957 study by the Bureau of Public Roads investigates driver behavior on two-lane primary rural highways, specifically examining how shoulder width, surface type, and visual contrast influence vehicle speed, lateral placement, and safety. The research was motivated by concerns that bituminous-paved shoulders matching the appearance of traffic lanes caused drivers to treat two-lane roads as four-lane highways, potentially compromising safety since shoulders are not designed to carry heavy traffic loads. The study analyzed data from 89,000 vehicles across 66 locations in nine Western states. Researchers categorized test sections into groups based on shoulder characteristics: gravel shoulders of varying widths, combination bituminous and gravel shoulders, and bituminous-paved shoulders that either contrasted with or matched the traffic lanes. Data collection involved recording vehicle speeds and lateral positions, classifying vehicles as free-moving, meeting oncoming traffic, or involved in passing maneuvers. The study also evaluated the impact of edge striping, including solid white and yellow stripes, on driver behavior. Key findings indicate that shoulder surface types did not significantly affect vehicle speeds, which averaged 55 mph for passenger cars and 48 mph for commercial vehicles. However, lateral placement was heavily influenced by visual contrast. On sections where shoulders matched the traffic lanes, commercial vehicles encroached on the shoulder nearly 80 percent of the time when meeting other vehicles, compared to one-third to one-half that rate on sections with contrasting shoulders. Slower vehicles tended to utilize the full width of matching paved surfaces more than faster vehicles. Clearances between meeting vehicles were widest on matching sections (averaging over 10 feet) and narrowest on gravel shoulders (approximately 6 feet). Additionally, matching shoulders encouraged passing maneuvers, resulting in 30 percent more passing events compared to other configurations. The application of a 2-inch solid white edge stripe reduced shoulder encroachment by approximately 50 percent on matching sections. The study concludes that visual distinction between traffic lanes and shoulders is critical for confining vehicles to the traveled lanes. While matching shoulders increased the usable pavement width, they led to excessive shoulder use by commercial vehicles and altered passing behavior. The results support the practice of maintaining a clear visual contrast between lanes and shoulders or using edge striping to define lane boundaries, ensuring that drivers do not treat shoulders as additional travel lanes.

Key finding

Shoulder encroachment by commercial vehicles reached nearly 80 percent on matching bituminous-paved sections, whereas a 2-inch solid white edge stripe reduced such encroachment by about 50 percent.

Methodology

naturalistic

Sample size: 89000

Provenance

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

Topics

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• signage environment
• roadway lighting effects
• traffic density

Information type

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• Empirical Findings: observational prevalence