Superelevation Criteria for Sharp Horizontal Curves on Steep Grades
Record Type: RiP
Sharp horizontal curves on steep grades represent a particularly dangerous situation for vehicle operators, especially heavy vehicle operators. Examples where this combination may occur are high-speed interchange movements, switchback curves on mountainous two-lane, two-way roads or high-speed downgrade curves on limited access roadways. At these locations, the complicating factors of vehicle off-tracking, pavement slope, and pavement friction fully tax the driver's ability to provide correct vehicle positioning without compromising control of the vehicle. Accident problems have arisen where, as a result of reconstruction, older highways with 12% to 17% superelevation have been rebuilt using 8% and 10% superelevation in accordance with current standards. Superelevation criteria, and other associated horizontal curve criteria, for situations where steep grades are located on sharp horizontal curves have not been developed. NCHRP Projects 15-16 and 15-16A, documented in NCHRP Report 439: Superelevation Distribution Methods and Transition Designs, evaluated and recommended revisions to the horizontal curve guidance presented in the 1994 AASHTO publication, A Policy on Geometric Design of Highways and Streets (Green Book). The two principal design elements evaluated were the use of superelevation and the transition from a tangent to a curve. The transition recommendations were incorporated into the 2001 edition of the Green Book and the superelevation recommendations were included in the 2004 edition of the Green Book. NCHRP Report 439 noted that significant roadway downgrades deplete the friction supply available for cornering. This depletion results from the use of a portion of the friction supply to provide the necessary braking force required to maintain speed on the downgrade. The report found that both upgrades and downgrades yield an increase in side friction demand and a decrease in side friction supply. This undesirable combination results in a significant decrease in the margin of safety resulting from roadway grade, especially for heavy vehicles. Superelevation criteria and horizontal curve criteria for this situation were not developed. >The 2004 Green Book contains the following: "On long or fairly steep grades, drivers tend to travel faster in the downgrade than in the upgrade direction. Additionally, research has shown that the side friction demand is greater on both downgrades (due to braking forces) and steep upgrades (due to the tractive forces). Some adjustment in superelevation rates should be considered for grades steeper than 5%. This adjustment is particularly important on facilities with high truck volumes and on low-speed facilities with intermediate curves using high levels of side friction demand.” The 2004 Green Book further states that this adjustment for grade can be made by assuming a slightly higher design speed for the downgrade and applying it to the whole traveled way. There are no guidelines as to how this adjustment should be made for two-lane or multilane undivided roadways. More definitive guidance on this adjustment, as well as adjustment for other elements of the horizontal curve, is needed. The objective of this research is to develop superelevation criteria for horizontal curves on steep grades. Other criteria associated with design of horizontal curves such as tangent-to curve transitions, spiral transitions, lateral shift of vehicles traversing the curve, need for pavement widening, and minimum curve radii should also be considered in the development of the criteria. The criteria may be based on quantitative evidence obtained from theoretic considerations and simulations but should be supported by actual field observation. The research should include a review of current practice, development of a work plan to achieve the research objectives, collection of data and other information, evaluation of effects of various alternatives and candidate criteria, and preparation of final criteria. The recommended criteria should be documented in the final report and also presented in a form that could be used by the AASHTO Technical Committee on Geometric Design in a future edition of the Green Book. This research topic was selected by the Transportation Research Board (TRB) Committee on Geometric Design, TRB Committee on Operational Effects of Geometrics, and the AASHTO Technical Committee on Geometric Design at their combined meeting in June, 2004 as one of the five highest priorities for research. The research is needed immediately to fill a gap in current superelevation design policy. The superelevation guidance will apply to high speed interchange ramp alignments on descending grades. As such, the research findings will have applications in every State and not just to those with mountainous terrain. Considering the research will apply to interchange movements, this research topic will be of use in the design of highways nationwide.
Start date: 2010/4/10
Contract/Grant Number: Project 15-39
Total Dollars: 500000
Source Organization: National Cooperative Highway Research Program (NCHRP)
Notes: Contract to a Performing Organization has not yet been awarded.
Date Added: 05/18/2009
Index Terms: Superelevation, Steering, Highway curves, Switchbacks, Steep grades, Highway design, Traffic accidents, Geometric design, Downgrades (Roads),
National Cooperative Highway Research Program (NCHRP)
Transportation Research Board
500 Fifth Street, NW
Washington, DC 20001
American Association of State Highway & Transportation Officials
444 North Capitol Street, NW, Suite 225
Washington, DC 20001
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
Phone: (202) 366-4000
Reynaud, David A.