Thoughts on a New Approach for
Signing Roadway Curves
John C. Glennon, D. Engr., P.E.
January 2003 (copyright)
The U.S. has thousands of substandard roadway curves that became more dangerous in 1996 as speed limits were increased across the nation. On average, roadway curves have five times the accident rate of straight roadways.1 And, sharper roadway curves have higher accident rates than flatter roadway curves.
Now in a new millennium with rapidly expanding technology, the U.S. still employs archaic policies for signing roadway curves. These policies vary by jurisdiction from a complete lack of any applied science to the determination of the controlling design (safe) speed based on a weak premise using an inaccurate measurement technique (Ball- Bank Indicator).
The 1988 Manual on Uniform Traffic Control Devices (MUTCD)2 was intended to ensure uniform application of traffic control devices nationwide. Yet, when it addressed signing for roadway curves, the 1988 MUTCD through its ambiguous and ambivalent language actually promoted widely varying practices. The result of applying this practice was that drivers were sometimes surprised by sharp roadway curves that had no warning at all, and, at other over-signed roadway curves, drivers could easily operate 15-20 mph faster than the advisory speed. For example, some (usually local) agencies failed to sign 20-mph (safe speed)roadway curves hidden over sharp hillcrests on 55-mph roadways, while other (usually State) agencies installed advance warning signs, 50-mph advisory speed plates, and chevron signs at 54-mph (safe speed) roadway curves on 55-mph (speed limit) roadways.
The new 2001 MUTCD3 may have slightly improved the application of roadway curve signing. Although it still does not give clear guidance, it does have some new useful signs and does more strongly emphasize the need for engineers to make signing decisions. So the opportunity exists now to develop a more objective engineering basis for making proper and uniform decisions about roadway curve signs.
What is needed is a uniform application of signs based on a clear-cut analysis of both the dynamics of vehicle cornering and the human tolerability of drivers to the higher lateral forces generated in cornering. This paper suggests an analytical development of lateral acceleration criteria for determining each application of curve (or turn) warning signs, advisory speed plates, and/or chevrons. The desired result of this analysis is a recommended uniform approach for applying these signs nationwide.
Some roadway agencies and/or some of their subdivisions use very few roadway curve signs where they are truly needed, while other agencies and/or their subdivisions use a plethora of signs and unrealistically low advisory speeds where they are not needed. Because the practice of signing roadway curves can vary so much not only between nearby jurisdictions but also within jurisdictions, drivers, on one hand, can lose respect for the signs and, on the other hand, can be surprised by unmarked sharp roadway curves.
Those agencies that have definitive warrants for signing roadway curves will have a practice similar to that shown in Figure 1.4,5 This practice avoids any signing when the safe speed equals or exceeds the speed limit, uses an advance warning sign when the safe speed of the road curve is from 10-14 mph below the operating speed of the roadway, and adds an advisory speed plate reflecting the nearest 5-mph speed increment at or below the safe speed when the deviance is 15 mph or greater.
This kind of practice ignores several factors related to the dynamics of the vehicle and driver tolerances to lateral acceleration, as follows:
A suggested process for achieving a more realistic basis for signing roadway curves might include the following objectives:
The philosophy promoted here is that, the higher the expected level of friction demand for a combination of roadway curve geometry and vehicle operating speed, the greater the need to communicate to the driver through traffic control devices. In other words, once a design vehicle speed and vehicle path can be related to the roadway curvature, length, and superelevation, a design level of lateral acceleration can be determined for signing that roadway curve. Likewise, a rationale is needed for determining the criticality of various levels of lateral acceleration such that a table similar to Table 1 can be developed.
The following tasks are suggested for developing a more objective and uniform basis for signing roadway curves:
1. Neuman, Timothy R., Glennon, John C., and Saag, James B., Accident Analysis for Highway Curves, Transportation Research Record 923, 1983.
2. Federal Highway Administration, Manual on Uniform Traffic Control Devices, 1988.
3. Federal Highway Administration, Manual on Uniform Traffic Control Devices, 2001.
4. Kansas Department of Transportation, Handbook of Traffic Control Practices for Low Volume Rural Roads, 1991.
5. Federal Highway Administration, Traffic Control Devices Handbook, 1983.
6. Kummer, H. W., and Meyer, W. E., Tentative Skid Resistance Requirements for Main Rural Highways, Highway Research Board, NCHRP Report 37, 1967.
7. Glennon, John C., Neuman, Timothy R., and McHenry, Brian G., Prediction of the Sensitivity of Vehicle Dynamics to Highway Curve Geometrics Using Computer Simulation, Transportation Research Record 923, 1983.
8. Glennon, J. C., Neuman, T. R., and Leisch, J. E., Safety and Operational Considerations for Design of Rural Highway Curves, Federal Highway Administration, 1983.
9. Glennon, John C., and Weaver, Graeme D., Highway Curve Design for Safe Vehicle Operations, Highway Research Record 371, 1972.
10. Glennon, John C., Effect of Alinement on Highway Safety, Transportation Research Board, State of the Art Report 6, 1987.
11. Glennon, John C., and Weaver, Graeme D., The Relationship of Vehicle Paths to Highway Curves, Texas Transportation Institute, Research Report No. 134-5, 1971.
12. Perchonak, K., et. al., Methodology for Reducing the Hazardous Effects of Highway Features and Roadside Objects, Federal Highway Administration, 1978.
13. Messer, C. J., et. al., Highway Geometric Design Consistency Related to Driver Expectancy, Federal Highway Administration, 1981.
14. Taylor, W. C., and Foody, T. J., Curve Delineation and Accidents, Ohio Department of Highways, 1968.
15. Glennon, John C., State of the Art Related to Safety Criteria for Highway Curve Design, Texas Transportation Institute, Research Report No. 134-4, 1969.
About the Author
Dr. John C. Glennon is a traffic engineer with over 45 years experience. He has over 120 publications. He is the author of the book "Roadway Safety and Tort Liability" and is frequently called to testify both about roadway defects and as a crash reconstructionist.
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