Public Roads: A Journal of Highway Research, Vol. 21, No. 10

Barber, E. S.; Palmer, L. A.; Walker, W. P.; Mizroch, Jacob; Rapp, Paul · 1940 · ROSA P / United States. Government Printing Office

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Summary

This 1940 issue of *Public Roads* presents two distinct studies conducted by the Public Roads Administration regarding highway engineering and safety. The first study, by L. A. Palmer and E. S. Barber, addresses the evaluation of subgrade supporting characteristics under flexible pavements. The second, by W. P. Walker, investigates the effects of highway lighting on driver behavior. Palmer and Barber propose a method to compute vertical soil displacement (settlement) due to lateral yield under a uniform load over a circular area. The approach combines laboratory-determined stress-deformation curves from triaxial compression tests with rational theoretical analyses based on elasticity theory. Instead of using the modulus of elasticity, the authors utilize a "modulus of deformation" ($C$), defined as the ratio of stress to deformation regardless of whether the deformation is elastic or plastic. The study assumes settlement occurs at constant volume, implying a Poisson’s ratio of 0.5. The authors derive formulas for vertical displacement using a "settlement factor" ($F$) dependent on depth and load radius. They demonstrate that the shape of the loaded area has little effect on settlement, with circular, square, and rectangular areas yielding similar average settlements. Through examples involving clay soils and various pavement thicknesses, the study shows that increasing pavement thickness reduces settlement, though the effect diminishes as thickness exceeds twice the load radius. The findings suggest that this theoretical method, validated by future large-scale tests, offers a promising alternative to direct penetration tests for designing flexible pavements. Walker’s study examines driver behavior on a one-mile section of U.S. Route 422 near Chagrin Falls, Ohio, comparing daytime, nighttime with lighting, and nighttime without lighting conditions. Using specialized equipment to record passing practices, transverse vehicle positions, and speeds, the study found that driver behavior on lighted highways at night closely mirrored daytime behavior. Specifically, drivers utilized 55.6 percent of available passing opportunities on lighted roads at night, compared to 57.7 percent during the day. In contrast, on unlighted roads at night, passing utilization dropped to 38.5 percent. Transverse positioning also remained consistent between daytime and lighted nighttime conditions, with vehicles averaging 3.3 feet from the pavement edge. However, on unlighted roads, vehicles moved approximately one foot closer to the road center. The study concludes that highway lighting effectively restores safe driving behaviors to levels comparable to daylight, thereby reducing the hazards associated with nighttime driving.

Key finding

Drivers utilized 57.7 percent of available passing opportunities during daylight, 55.6 percent on lighted highways at night, and only 38.5 percent on unlighted highways at night.

Methodology

mixed_methods

Provenance

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

Topics

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• roadway lighting effects