Public Roads: A Journal of Highway Research and Development, Vol. 55 No. 3

Chowdhury, Mashrur A.; Warren, D.L.; Bissell, Howard; Virmani, Yash Paul; Wright, William; Nelson, Ronald N.; Stuart, Kevin; Mogawer, Walaa · 1991 · ROSA P / United States. Government Printing Office

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Summary

This document presents two distinct research studies published in *Public Roads*, Vol. 55, No. 3 (1991), addressing highway safety criteria and material testing standards. The first study, by Chowdhury, Warren, and Bissell, investigates the validity of current advisory speed setting criteria for highway curves. The second, by Virmani, Wright, and Nelson, evaluates the fatigue characteristics of Thermex reinforcing bars compared to conventional steel. The advisory speed study was motivated by evidence that posted speeds are often set too low and inconsistently, leading to poor driver compliance. The authors evaluated 28 curves in Virginia, Maryland, and West Virginia, collecting data on geometry, spot speeds, and ball-bank readings. They compared observed driver behavior against existing criteria, which relied on ball-bank indicators and side friction factors derived from 1930s tests. Results showed that 90% of motorists exceeded posted advisory speeds, with average exceedances of 14.5 km/h (9 mi/h). The study found that current ball-bank criteria (10°–14°) and side friction factors (0.16) were too conservative for modern vehicles. Observed 85th-percentile speeds corresponded to ball-bank readings 5° to 10° higher than current standards and side friction factors nearly twice as high (0.29 vs. 0.16). The authors concluded that criteria should be revised upward, suggesting ball-bank readings of 12°–20° and side friction factors of 0.20–0.30 to better reflect actual driver behavior and vehicle capabilities. The second study addressed the lack of standardized fatigue testing for reinforcing steel in the U.S., specifically comparing Thermex bars (produced via water quenching and self-tempering) with conventional alloyed bars. The Federal Highway Administration conducted axial fatigue tests on #5, #6, and #11 bars using a servo-hydraulic load frame. A significant portion of the research focused on resolving "grip problems," where failures occurred at the test machine grips rather than the bar itself. The authors tested four gripping methods: shims, machined ends, babbitt metal embedding, and high-strength grout embedding. Grouted conical grips proved most effective for producing valid failures away from the grip area, particularly for Thermex bars. Fatigue testing revealed no significant difference in fatigue resistance between Thermex and conventional bars, with both performing above the lower 95-percent confidence limit of existing design lines. However, the study noted a large discrepancy between U.S. test results and stricter British standards, highlighting the need for a unified U.S. specification for rebar fatigue testing.

Key finding

Drivers exceeded posted advisory speeds by an average of 14.5 km/h (9 mi/h), and existing ball-bank criteria produced advisory speeds significantly lower than prevailing traffic speeds.

Methodology

on_road

Sample size: 28

Provenance

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Topics

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• road surface friction