Speed and Design Consistency of Combined Horizontal and Vertical Alignments in Two-Lane Rural Roads

The American Association of State Highway and Transportation Officials (AASHTO) establishes that the highway design speed should be logical with respect to the anticipated operating speed, the topography, the adjacent land use, and the functional classification. The AASHTO equation for the minimum horizontal curve radius for a given design speed depends on the combination of the superelevation rate and the side friction factor. This equation provides a balance of forces acting on a vehicle traversing on a circular path for a given speed, but does not consider the effect on the actual speed and the safety performance of an overlap between horizontal and vertical curves. The presence of the longitudinal grade in horizontal curves tends to increase the risk of crashes because it affects the driver's perception of the horizontal curvature. The objective of this investigation is to study the influence in safety and operating speeds when a horizontal curve is combined with a vertical curve. The study will identify the relationship between the combined horizontal and vertical alignment conditions, operating speeds, and safety, used in the design consistency assessment of two-lane rural highways. A sampling of two-lane rural roads will be performed to identify horizontal curves overlapped with vertical curves in Puerto Rico. Roadway geometry, free-flow speed and crash data will be collected for the selected sites. The speed data will be collected at different points along the horizontal curve with the use of portable traffic classifiers and vehicle-tracking speed guns. The radius of horizontal curves has been identified as one of the most relevant highway features in influencing operating speeds (driving behavior). Most of the earlier studies on speed prediction and design consistency focused on isolated horizontal alignment conditions. One of the expected results is a comprehensive review of recent studies that have explored the issue of combined horizontal and vertical alignment and design consistency. It is anticipated that the recent literature will demonstrate the need for a speed prediction model for different types of curves radius and vertical grades. Another anticipated result is the development and calibration of a curve speed model that considers the geometric design of combined horizontal and vertical alignments. This model could serve to update current geometric design practices and the AASHTO horizontal curve design equation. In addition, the speed and crash data could serve to update speed and crash prediction models for two-lane rural roads that could enhance the roadway assessment tools included in the Highway Safety Manual and Safety Analyst.


  • English


  • Status: Completed
  • Funding: $84335.00
  • Contract Numbers:


  • Sponsor Organizations:

    Research and Innovative Technology Administration

    University Transportation Centers Program
    1200 New Jersey Avenue, SE
    Washington, DC  United States  20590
  • Project Managers:

    Mooney, Deborah

  • Performing Organizations:

    University of Puerto Rico, Mayaguez

    University of Puerto Rico - Mayaguez Campus
    Mayagüez, PR  United States  00681
  • Principal Investigators:

    Papadopoulos, Chris

    Cruzado Velez, Ivette

  • Start Date: 20121001
  • Expected Completion Date: 0
  • Actual Completion Date: 20140331
  • Source Data: RiP Project 33075

Subject/Index Terms

Filing Info

  • Accession Number: 01480662
  • Record Type: Research project
  • Source Agency: University Transportation Research Center
  • Contract Numbers: 49997-39-24
  • Files: UTC, RIP
  • Created Date: May 3 2013 1:00AM