This research will update appropriate values (such as side friction, superelevation, e and f distribution) 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 they were reviewed in 2000 under NCHRP Report 439, which 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 is needed. More sophisticated vehicle dynamics simulation models also have 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 technologically advanced, the newer vehicle models may be able to refine acceptable driver comfort as it relates to horizontal curvature within superelevation transition areas and when the curve is fully superelevated. The vehicle fleet now includes connected and automated vehicles (CAVs). As the automation technology increases, the vehicles will become self-driving. This research should examine how this change in the vehicle fleet may impact horizontal curves, cross slopes, and vehicle models for such design elements. The research should show how the AASHTO current design vehicle(s) would compare to the proposed design vehicle(s).
Potential tasks include the following.
1. Completion of a comprehensive literature review to 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, and identify available vehicle models that may be candidates to replace the point-mass model.
2. Evaluate the current research and identify which components need additional research. This could include tire performance/friction factors (wet and dry), pavement wearing surface (type and friction in combination with tire performance), vehicle fleet (which vehicles to use in design: keep two primary categories of heavy vehicles/trucks and everything else, or some other classification, traditional human driven vehicles, CAV with Automation Level up to 3, mixed fleet: human driven and CAV, autonomous vehicles: Automation Level 4/5, stability capabilities).
3. Cross-slope constructability (is 0.2 the appropriate interval for design superelevation? NYSDOT has adopted 0.5 increments).
4. Vehicle operator (should the design parameters be modified if there is a high presence of “older” drivers and what would quantify “high presence,” CAVs.
5. Perform the research based on identified needs and make recommendations (revised side friction factors for use in design, revised design superelevation rates, superelevation transitions, appropriate vehicle model, propose new text for the next edition of the AASHTO Green Book’s Chapter 3, Elements of Design.
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. Additionally, vehicle technologies such as electronic stability control have changed vehicle dynamics and may influence speeds that drivers are comfortable at while traversing horizontal curves. Since NCHRP Report 439: Superelevation Distribution Methods and Transition Design was published, there have been findings that superelevation following NCHRP Report 439 transitions may cause greater side forces than expected. More sophisticated vehicle dynamics simulation models are 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 and TRB Special Report 214: Designing Safe Roads: Practices for Resurfacing, Restoration, and Rehabilitation have shown that the minimum radii in AASHTO’s Green Book are not threshold values that correlate to safety or operational problems.
