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FHWA Highway Safety Programs

2. Step 2: Conduct Field Verification and Design

Field verification is an important step in determining whether HFST could be implemented, and if implemented, the limits of the HFST installation. Below is a check list of items that may be considered during a field review.

  • Select an Appropriate Timeframe. Consider scheduling field reviews to correlate with the primary conditions of collision patterns: time of day, day of week, season (e.g., winter, summer), pavement conditions (e.g., wet, dry), light conditions (e.g., dark, daylight), etc.
  • Physically Locate Crashes. Bring a populated crash report diagram so that crashes within the curve can be easily located and studied during the field visit. Having a better understanding of crash contributing factors and location within the curve can help determine whether HFST is a viable solution and identify the limits of implementation.
  • Record Roadway and Traffic Characteristics. Drive through the location several times to observe roadway and traffic characteristics, driver responses, signs, signals, and pavement markings. Observe drivers to determine behavior at the location. Take photo and/or videos and maintain a log of all photos and videos taken: location, direction, date and time of photo/video, photographer's name, etc. When possible, take a series of panoramic photos. Measure, document, and confirm a variety of data such as:
    • Presence of skid marks or trees missing bark where vehicles have made impact within the curve;
    • Presence and condition of previous low-cost countermeasures (e.g., signing, pavement markings);
    • Damage to roadside barriers;
    • Superelevation;
    • Speed advisory measurements;
    • Point of Curvature (PC) and Point of Tangent (PT ) of the horizontal curve;
    • Horizontal and vertical sight distances;
    • Intersections near or within the curve;
    • Whether any of the "curve-relate" crashes are incorrectly categorized and may actually be intersection-related;
    • Heavy vehicle use (especially where vehicles frequently use winter tire chains); and
    • Speed differentials between the advisory speed and the posted speed limit.
  • Determine HFST Start and End Points. Identify exact limits of the application if HFST is chosen as a safety countermeasure. Typically, HFST is installed at the point where vehicles start to brake. Brake lights are a good indication of where treatment could start, as the intention is to slow down the drivers as they are going into the curve. Many agencies end their treatment at the PT.17 The Texas Transportation Institute published a paper that provides further recommendations on how to select start and end points for HFST installation.18
  • Conduct a Friction Test. Lower friction values have been shown to increase crash frequency. Typical means to determine friction include the British Pendulum Test (BPT ), the Dynamic Friction Tester (DFT ), the Griptester, and the Locked Wheel Skid Trailer.19 While agencies set different thresholds for satisfactory pavement friction, it is generally accepted practice to focus on pavement sections where skid numbers are below 30.
  • Check Pavement Condition. If not already known, collect a pavement core sample to determine the pavement's composition, age, and quality. Also, conduct a visual inspection and record any defects in the pavement that will need to be repaired prior to HFST installation (e.g., cracks, pot holes, unstable joints, etc.). A pavement condition rating of "Good" or better is needed prior to placing HFST.20
  • Screening Sample Questions. These questions may be considered while conducting a field review. The answers to some may indicate that HFST is not the best solution. Due to the site-specific nature of some of the questions, they cannot be applied universally.
    • Are the collisions related to physical conditions of the location?
    • Are existing signs, delineation, and pavement markings accomplishing their purpose? (I.e., Are there too few or in poor shape? Are they visible?)
    • Are there any specific traffic movements to be prohibited or favored?
    • Is special nighttime enhancement needed? Is existing lighting satisfactory? Are existing signs and delineation visible and retro-reflective at night?
    • Are selective enforcement or maintenance procedures needed?
    • Is parking contributing to collisions? Is sight distance impaired by the parking?
    • Are there adequate advance guide signs so that motorists may choose proper lanes and directions well in advance of need?

17 FHWA's High Friction Surface Treatments Frequently Asked Questions. Publication No. FHWA-CAI-14-019. [ Return to note 17. ]

18 B. Brimley and P. Carlson, Using High Friction Surface Treatments to Improve Safety at Horizontal Curves. July 2012. Available at: http://d2dtl5nnlpfr0r.cloudfront.net/tti.tamu.edu/documents/TTI-2012-8.pdf. [ Return to note 18. ]

19 American Traffic Safety Services Association, Safety Opportunities in High Friction Surfacing. February 2013. [ Return to note 19. ]

20 Oregon Department of Transportation, GFP Pavement Condition Rating Manual. July 2010. Available at: http://www.oregon.gov/odot/hwy/construction/docs/pavement/gfp_manual.pdf. [ Return to note 20. ]