There are few CMFs available related to HFST at this time, but data collection efforts are ongoing.
The FHWA Turner-Fairbank Highway Research Center (TFHRC), as part of the Evaluations of Low Cost Safety Improvements Pooled Fund Study, published the report “Evaluation of Pavement Safety Performance,” which included some preliminary CMFs. The report indicated the results for the before-after study for treatment sites with adjustment using Highway Safety Manual CMFs correction factor yield a CMF for total crashes of 0.653 for ramps and 0.385 for curves, and a CMF for wet-road crashes of 0.139 and 0.481 for ramps and curves. Additional data collection efforts are also underway, including studies of additional control sites to develop more robust CMFs and benefit-cost ratios. The evaluation report is available at: https://www.fhwa.dot.gov/publications/research/safety/14065/14065.pdf
The Pennsylvania, Kentucky, and South Carolina DOTs report a before/after total crash reduction of 100 percent, 90 percent, and 57 percent, respectively, for their signature trial projects, for which the after periods were from three to five years. Kentucky went on to install and monitor 60 additional HFST applications during the 2010-2012 period; these sites are showing total crash reductions of 78 percent for the group, with wet-weather crash reductions of 85 percent. Additional analysis was performed in early 2015 and these sites are showing total crash reductions of 80 percent, with wet-weather crash reductions of 87 percent and dry-weather crash reductions of 58 percent.
HFST should be considered when delineation treatments are not reducing crashes, and pavement friction is suspected to be inadequate for prevailing conditions. Generally, signs and markings are effective for prudent drivers who are trying to drive carefully, but when vehicles enter a curve too fast it is generally too late for those devices to help the driver. Whether some drivers are willfully speeding or distracted and the curve surprises them, speed is a major contributing factor for crashes in curves. Many studies have shown that a pavement with high skid resistance can give drivers a “hidden advantage” and help them still navigate the curve. While everything has a practical limit, in general, studies have shown that doubling the friction reduces the crashes by half.
If the pavement is polished or has lower skid resistance to begin with, safely navigating the roadway can become a problem even for prudent drivers, particularly when the pavement is wet. Therefore, placing HFST provides a pavement that is polish-resistant and has excellent friction. Otherwise, safety enhancements may require geometric improvements, which can take a long time, be expensive, increase delay and safety risks associated with lengthy active work zone periods, and have environmental consequences. By comparison, HFST costs less and has many fewer impacts.
HFST provides one of the best long-term life-cycle costs available due to its durability and positive impact on safety. HFST has the potential to provide (and in many instances is already providing) a higher crash reduction rate than would be generally expected with lower cost safety treatments such as chevrons and rumble strip applications on horizontal curves. As mentioned previously, several States are experiencing large crash reductions of 50-100 percent after application of an HFST at specific sites. In addition, National Cooperative Highway Research Program (NCHRP) Report 617 indicates a crash reduction of 20 percent for all intersection crashes. Please refer to the FHWA Office of Safety – Roadway Departure, Pavement Friction webpage for case studies on crash reduction benefits: https://safety.fhwa.dot.gov/roadway_dept/pavement_friction/case_studies_noteworthy_prac/
There are no known negative safety effects for motorcycles and bicycles. In fact, braking and cornering on a motorcycle demand more friction than similar maneuvers do for passenger vehicles, so HFST can be even more impactful in improving motorcycle safety on curves and intersection approaches. The United Kingdom uses HFST as a safety countermeasure to specifically address motorcycle crashes at these locations. In addition, a colored version of HFST has been used for the treatment of many miles of bike lanes in the USA.
No increase in rollovers has been reported from any HFST installations, and studies by FHWA in the past have shown that increases in friction help reduce truck rollover.
Because HFST is applied only at spot locations for smaller areas, excessive tire wear should not be an issue. Getting people home safely far outweighs tire wear as a concern.