Sharp, horizontal curves on steep downgrades represent a potential safety concern for vehicles, especially heavy vehicles. Examples where this combination may occur are interchange ramp movements, curves on mountainous roads, or high-speed downgrade curves on controlled access roadways. At these locations, the complicating factors of grade, pavement cross slope, and pavement friction fully tax the driver's ability to provide correct vehicle positioning without compromising control of the vehicle. Superelevation criteria, horizontal curvature, and other associated geometric criteria have not been developed for situations where steep grades are located on sharp, horizontal curves. 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 in 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 some of the friction supply to provide the braking force required to regulate speed on the downgrade. The report found that both upgrades and downgrades increase side friction demand and decrease side friction supply. This combination significantly decreases 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, and 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 (e.g., tangent-to-curve transitions, spiral transitions, lateral shift of vehicles traversing the curve, need for pavement widening, and determination of curve radii) may also need to be considered. The criteria may be based on quantitative evidence obtained from theoretical considerations and simulations, but should be supported by actual field testing. The recommended criteria should be documented in the final report and also presented in a form that can be used by the AASHTO Technical Committee on Geometric Design in a future edition of the Green Book.
(1). Conduct a review of literature and current practice pertaining to the research objective. (2). Identify the critical parameters, including the type of vehicle that may contribute to compromising the control of vehicles on sharp, horizontal curves on steep grades. (3). Suggest a framework for an analytical model incorporating these critical parameters. (4). Develop a method, based on field observations or testing, for calibrating the proposed model. (5). Prepare an interim report on the information developed in Tasks 1 through 4. The interim report shall also contain an updated, detailed work plan for field testing in Phase II. The research plan shall provide a 1-month period for review and approval of the interim report. An interim meeting of the project panel to discuss the report with the research agency will be required. The research agency shall not begin work on the remaining tasks without NCHRP approval.
(6). Develop the model and collect pertinent field and test data to calibrate/confirm the model. Revise the model as appropriate. (7). Develop recommended criteria in an appropriate format for possible inclusion in the Green Book. (8). Submit the final report documenting the entire research project. The recommended criteria shall be presented in a form suitable for consideration in future editions of the Green Book.