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Advanced Modeling of Driver Performance on Horizontal Curves


The side friction factors may need to be adjusted due to multiple factors. Both tires and road wearing surfaces have changed, which affect the friction between them. Since NCHRP Report 439 (Superelevation Distribution Methods and Transition Design) was published, there have been findings that superelevation following the NCHRP 439 transitions may cause greater side forces than expected. More sophisticated vehicle dynamics simulation models are also now available that provide advantages over using the point-mass model for design, such as accounting for grade, deceleration/acceleration, and analysis of the dynamics of individual axles.

Changing the design superelevation from increments of two tenths, which may be too precise, needs to be considered. What precision can contractors actually construct? Many believe this is not practical to contractors. Considering performance-based design principles, are there acceptable differences in superelevation from the current design superelevation rate that would be acceptable? Could the design speeds for the various design superelevation rates be modified? The side friction factor also represents the lateral acceleration, so the way lateral acceleration is modeled in horizontal curve design may need to be changed. Since the previous research was completed, there are different vehicle types in the vehicle fleet and a departure from the point-mass model. Are there better models that should be used that work better with the current fleet of vehicles? If there are better models, which should be used and how does that change the equations used for design?

The AASHTO Highway Safety Manual, the collective experiences of state transportation departments, and TRB Special Report 214 have shown that the minimum radii in AASHTO’s Green Book are not threshold values that correlate to safety or operational problems. Based on NCHRP Report 774 (Superelevation Criteria for Sharp Horizontal Curves) and NCHRP Report 439:

· The current approach for selecting the maximum side friction factors for design is based upon research from the 1930s and 1940s.

· Wet-tire side friction values are much higher than the AASHTO side friction factors used in design.

· The margin of safety for rollovers is variable since the current side friction factors used in design are more conservative at high speeds.


This research will update appropriate values for use in horizontal curve design. Multiple advancements have affected previous data and models that have been used to establish horizontal curve design guidance. Side friction factors were established in the 1930s and 1940s, and then they were reviewed in 2000 under NCHRP Report 439 and found the values to be generally consistent with the prior values. With advancements in tire design and manufacturing and pavement wearing surfaces and mixes continuing to develop, a reexamination of appropriate side friction factors for use in design need to be examined. More sophisticated vehicle dynamics simulation models have also been developed to model the behavior of vehicles in curves. Current Green Book horizontal curve design is based on the point-mass model. As the vehicle fleet has advanced, the newer vehicle models may also be able to refine acceptable driver comfort as it relates to horizontal curvature within superelevation transition areas and when the curve is fully superelevated.


This research is important, because as our vehicular technology advances, so must our design criteria. We should use design criteria that is applicable to our current vehicle fleet. Designing to a prior vehicle fleet and old vehicular models may cause over design (additional costs) or under design (potential safety impacts). The benefits of updating the design criteria could be substantial. We would like to have the results of this research before the next edition of the “Green Book” is published.

This research topic was identified as a high priority by the AASHTO Technical Committee on Geometric Design, the TRB Committee on Geometric Design, and the TRB Committee on Operational Effects of Geometrics at their combined meeting in June 2018 from among a broad set of problems considered.

Related Research:

· Banney, T.A., and D.J. Segal. Evaluation of Horizontal Curve Design. United States Department of Transportation, Federal Highway Administration, 1980.

· Bauer, K..M., and D.W. Harwood. Safety Effects of Horizontal Curve and Grade Combinations on Rural Two-Lane Highways. United States Department of Transportation, Federal Highway Administration, 2014.

· Lamm, R., et al. Side Friction Demand Versus Side Friction Assumed for Curve Design on Two-Lane Rural Highways, Transportation Research Record 1303, 1991.

· Cruzado, I., et al. Speed and Design Consistency of Combined Horizontal and Vertical Alignments in Two-Lane Rural Roads. University Transportation Research Center, 2014.

· AASHTO, A Policy on Geometric Design of Highways and Streets, 6th Edition, 2011.

· Marvomatis, S., et al. “Investigation of Vehicle Motion on Sharp Horizontal Curves Combined with Steep Longitudinal Grades”. TRB Annual Meeting 2015. Paper 1578.

· Jung, S., et al. “Development of Highway Safety Policies by Discriminating Freeway Curve Alignment Features” Korean Society of Civil Engineers. 2018.

· Pratt, M., et al. “Validating Curve Speed Prediction Models Using Naturalistic Data”, TRB AHB65 sponsored paper. 2018.

· Bonneson, J. NCHRP Report 439 Superelevation Distribution Methods and Transition Design. TRB, 2000.

· Torbic, D.J., et al. NCHRP Report 774 Superelevation Criteria for Sharp Horizontal Curves. TRB, 2014.


Some of the tasks that could be completed in this project include:

· Completion of a comprehensive literature review.

o Identify research on vehicle fleet composition, performance of the vehicle, advanced technologies and their presence in the vehicle, percentage of vehicles with the advanced technologies (i.e., stability mechanics and other performance/safety innovations), tire / pavement friction based on current tires in production and typical pavement surface parameters. (Note this effort overlaps with another research needs statement – to evaluate potential revisions to acceleration and deceleration lengths.)

o Identify available vehicle models and which may be candidates to replace the point-mass model.

· Evaluate the current research and identify which components need additional research. This could include:

o Tire performance / friction factors (Wet & Dry)

o Pavement wearing surface (Types & Friction in combination with tire performance)

o Vehicle fleet

§ Which vehicles to use in design (i.e., keep two primary categories of heavy vehicles/trucks and everything else, or some other classification)

§ Stability capabilities

o Cross-slope constructability (Is 0.2 the appropriate interval for design superelevation? NYSDOT has adopted 0.5 increments.)

o Vehicle operator (Should the design parameters be modified if we have a high presence of “older” drivers? What would quantify “high presence”?)

· Perform the research based on needs identified above and make recommendations.

o Revised side friction factors for use in design.

o Revised design superelevation rates.

o Superelevation transitions.

o Appropriate vehicle model

· Propose new text based on the results of the research project for the next edition of the AASHTO Green Book. Revisions would be expected in what is currently Chapter 3 “Elements of Design”.


All engineers and agencies involved in the design of roads will be impacted by this research.

Sponsoring Committee:AKD10, Performance Effects on Geometric Design
Research Period:12 - 24 months
RNS Developer:David McDonald, Darren Torbic, Richard D. Wilder
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:Driver performance, Driving, Horizontal curvature, Highway curves, Side friction, Friction, Superelevation, Lateral acceleration, Highway safety, Highway design,
Cosponsoring Committees: 
Operations and Traffic Management
Safety and Human Factors

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