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Assessment and Updating of Horizontal Curve Design Criteria


Several recent developments make it timely to undertake a safety-based review of the current policy on horizontal curve design to assess whether modifications are needed to the design criteria in the AASHTO Policy on Geometric Design of Highways and Streets (commonly known as the Green Book) and, if so, what those revisions should be.

        There does not appear to be any need

to modify the point-mass model that forms the basis for horizontal curve design criteria in AASHTO policy. The issues that need to be addressed in the research include the following:

  • It is not clear whether the current AASHTO design criteria fully address the potential for hydroplaning on horizontal curves.

  • Recent research on driver comfort levels on horizontal curves appears to confirm the current friction factors (fmax) values in AASHTO policy, but this needs to be fully explored.

  • The recent National Transportation Safety Board (NTSB) investigation of a 2009 cargo-tank truck crash suggests the need for a review of the current AASHTO policy for pavement/shoulder cross-slope breaks on horizontal curves to determine whether any updates in design criteria are needed.

These three issues can be addressed in research that includes a stem-to-stern review of current horizontal curve design policies.



The purpose of this project is to identify various combinations of roadway design geometrics that, when constructed, could increase the potential for hydroplaning in wet weather conditions. Hydroplaning is a complex process affected by many variables including:

· roadway geometrics (pavement width, cross slope and grade);

· pavement texture (micro and macro);

· pavement condition (rutting, depressions, roughness);

· driver behavior, especially speed; and

· tire conditions (tread depth and inflation pressure)

Some roadway geometrics are preselected and/or beyond control of the designer. Also, urban drainage design, including any hydroplaning assessments, is often done after the roadway typical section, horizontal and vertical alignments and pavement surface types have been determined. Consequently, options available to the designer to reduce the flow path and accumulated depth of water on the pavement are limited.

This project will provide guidance for designers to assess hydroplaning potential earlier in the project development process thereby creating more opportunities to implement hydroplaning countermeasures.

It is envisioned that this research would provide a policy or planning tool rather than a project-specific design methodology. The research results could be used to demonstrate to policymakers the advantages and disadvantages of certain roadway typical sections in terms of hydroplaning potential. It could also be used to support certain geometric design guidelines such as the maximum number of lanes that can be sloped in one direction.

Friction Factors

Horizontal curves are designed so that drivers traversing a curve at the design speed will not experience lateral acceleration in excess of specific limits. These limits are presumed to provide substantial margins of safety against skidding and rollover. Current criteria were based on research conducted during the 1930s and 1940s. Recent research should allow the friction factors used in horizontal curve design (fmax) to be confirmed or updated.

Pavement/Shoulder Cross-Slope Breaks

The recent NTSB investigation of a 2009 cargo-tank truck crash at the interchange of I-69 and I-435 in Indianapolis, IN, suggests the need for a review of the current AASHTO policy for pavement/shoulder cross-slope breaks on horizontal curves to determine whether current policy is appropriate for passenger cars and for trucks. The current 0.08 ft/ft maximum cross-slope break was based on vehicle dynamics modeling which simulated the dynamic response of a 1971 passenger car only. NTSB has made specific recommendations for consideration by FHWA and AASHTO (http://www.ntsb.gov/news/events/2011/indianapolis_in/synopsis.html). Vehicle dynamics modeling is needed to determine whether the existing design policy is appropriate for the current fleet of passenger cars and trucks, including double-trailer trucks and trucks with high centers of gravity. In addition, careful consideration should be given to the frequency of crashes involving cross-slope breaks for various vehicle types, and the appropriate cross-slope break policies for interchange ramps, mainline freeways, and other roadway types, including the use of designs without a cross-slope break (i.e., letting the outside shoulder slope downward toward the traveled way), in a variety of climate conditions.

Related Research:


  1. Federal Highway Administration, Hydraulic Engineering Circular No. 22 (HEC 22). “Urban Drainage Design Manual,” 1996. FHWA-SA-96-078, p 4-4.

  2. American Association of State Highway and Transportation Officials, “2005 Model Drainage Manual,” AASHTO, Washington, D.C., 2005, Chapter 13.

  3. NCHRP Project 1-29, “Improved Surface Drainage of Pavements”, Final Report, Pennsylvania Transportation Institute, July 1998.

  4. American Association of State Highway and Transportation Officials, Highway Drainage Guidelines, Chapter 9, “Storm Drain Systems,” Task Force on Hydrology and Hydraulics, AASHTO, Washington, D.C., 2007.

  5. NCHRP. National Cooperative Highway Research Report Research Results Digest, Number 243: Proposed Design Guidelines for Reducing Hydroplaning on New and Rehabilitated Pavements. Transportation Research Board, Washington, DC, September 1999.

Friction Factors

  1. Tan, C.A., “An Investigation of Comfortable Lateral Acceleration on Horizontal Curves,” Ph.D Dissertation, Pennsylvania State University, 2005.

Pavement/Shoulder Cross-Slope Breaks

  1. http://www.ntsb.gov/news/events/2011/indianapolis_in/synopsis.html

  2. Glennon, J.C., T.R. Neuman, R.R. McHenry, and B.G. McHenry, HVOSM Studies of Cross Slope Breaks on Highway Curves, Phase II Task C, Report No. FHWA-RD-82-54, Federal Highway Administration, May 1982.

Other Sources to Consider in Overall Review of Horizontal Curve Design

  1. Bonneson, J.A., Superelevation Distribution Methods and Transition Designs, NCHRP Report 439, 2000.
  2. NCHRP Project 15-39, Superelevation Criteria for Sharp Curves on Steep Downgrades

Funding of the project is appropriate because the work would be completed in a timely fashion to be included in the next regular update of the AASHTO Green Book. The research responds directly to a recent NTSB finding that called on FHWA and AASHTO to undertake an assessment of cross-slope breaks and their interactions with high-center-of-gravity vehicles.

Sponsoring Committee:AKD10, Performance Effects on Geometric Design
Research Period:12 - 24 months
RNS Developer:Jeff Jones (TN DOT), Jim Rosenow (MN DOT), Douglas Harwood (MRIGlobal), Brian Toombs (Burgess & Niple)
Source Info:This problem statement was developed in connection with the June 2018 mid-year joint meeting of the AASHTO Technical Committee on Geometric Design, the TRB Committee on Geometric Design (AFB10), and the TRB Committee on Operational Effects of Geometrics (AHB65).
Date Posted:09/21/2018
Date Modified:12/31/2018
Index Terms:Friction factor, Cross slopes (Pavement surfaces), Hydroplaning, Design standards, Horizontal curvature, Highway curves, Geometric design,
Cosponsoring Committees: 
Operations and Traffic Management
Safety and Human Factors

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