AASHTO Asset Management Task Force
NCHRP 20-7
Proposed Research Needs Statement
AASHTO Technical Committee on Geometric Design
Jeff Jones, P.E.
October 10, 2016
TITLE
Reevaluation of the Perception and Reaction Time and Deceleration Rates for Stopping Sight Distance Using Crash Data
BACKGROUND / NEEDS STATEMENT
The AASHTO Subcommittee on Highways issued a resolution on Flexibility in Design Standards. The current minimum Stopping Sight Distances (SSDs) in AASHTO’s “A Policy for Geometric Design of Highways and Streets,” 2011 (a.k.a. Green Book) are not threshold values and substantial deviations can be made without impacting crash rates or severity.
The AASHTO Highway Safety Manual, the collective experiences of state transportation departments, and TRB Special Report 214 have shown that the minimum SSDs in AASHTO’s Green Book are not threshold values that correlate to safety or operational problems. For 2R and 3R projects, which use criteria based on cost-effectiveness, the SSD is often calculated using a speed 20 MPH below the design speed.
Based on NCHRP 400, “Determination of Stopping Sight Distances”:
• Most modern countries use a perception reaction time of 2 seconds instead of 2.5 seconds. This is closer to the 85th percentile of drivers than the 95th percentile used in the AASHTO policies.
• The deceleration rate of 11.2 ft/s2 is very conservative, and does not represent an emergency stop.
• The object height of 2’ is very conservative given the current fleet of vehicles with high mounted center taillights and the driver’s inability to recognize the need to stop for a 2’ high object at great distances.
• The 85th percentile driver’s eye height is higher as crossover, trucks, and SUVs represent a larger segment of the vehicle fleet and safety standards have resulted in much taller vehicles.
The figure below provides a comparison of SSD curves based on:
• the 2011 AASHTO Green Book;
• 2R/3R standards;
• Braking thresholds for worn tires on wet concrete pavement;
• A 2 second perception & reaction time; and
• Using twice the deceleration rate.
The very conservative SSD criteria result in lengthy minimum sight distances, which reduce the flexibility for the design and can result in additional costs, additional use of resources, and design exceptions with no apparent safety benefit. SSD is one of the 10 Controlling Design Parameters established by FHWA and should represent a safety threshold.
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RESEARCH OBJECTIVE
Determine a cost-effective SSD model for 2R/3R, Reconstruction, and New Construction Projects and revised text for an update of the AASHTO Green Book.
WORK TASKS
Tasks anticipated in this project include the following:
• Literature review, including NCHRP 400, Web only NCHRP 198, and NCHRP 774.
• Determine the threshold where crash rates and severity are impacted by SSD.
• Determine a cost-effective SSD model for 2R/3R, Reconstruction, and New Construction Projects.
• Provide revised text for an update to the AASHTO Green Book.
URGENCY
Funding and right-of-way constraints are a continual challenge for transportation facility owners. Additional flexibility is needed to help provide relief from overly conservative design standards. This research needs to be completed by the summer of 2017 to allow the results to be incorporated into the 2017/18 edition of the AASHTO Green Book.
FUNDING REQUESTED AND TIME REQUIRED
It is estimated that this research will take 9 months to complete and will require $50,000.
CONTACT PERSON
Richard D. Wilder, PE
Director, Design Services Bureau
New York State Department of Transportation
50 Wolf Road, POD 2-4
Albany, NY 12232
(518) 457-5922
Rick.Wilder@dot.
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