NOTEWORTHY PRACTICE 

TEXAS HIGH FRICTION SURFACE TREATMENT (HFST) 

Texas Department of Transportation’s Methodology for 

Selecting Curves to Receive High Friction Surface Treatments 

and Other Safety Improvements 

 What was the safety issue, problem, or gap?

The Texas Department of Transportation (TxDOT) explored ways to improve

horizontal curve safety by identifying whether geometric improvements or

pavement friction restorations would best address safety concerns associated

with a particular curve. TxDOT wanted to maximize the investment’s benefit

and minimize the need for additional improvements.
 

 What were the key challenges that needed to be addressed 

before the new practice could be implemented? 

Though it was anecdotally recognized, the interaction and effect among

geometric factors (e.g., radius, superelevation rate, and deflection angle),

vehicle speed, and available pavement friction were not completely

understood. As such, recognizing how these features worked together – or

in contrast to one another – would aid practitioners in selecting a safety

countermeasure tailored to the site.
 

Describe the new practice: 

TxDOT, in conjunction with the Texas A&M Transportation Institute (TTI),

developed a tool to assess the “margin of safety” for a particular horizontal

curve. The tool is known as the Texas Curve Margin of Safety, or TCMS.

Thinkstock “Margin of safety” is the “friction demand subtracted from friction supply;” in 

other words, the available roadway friction subtracted by the amount of friction 

needed by a vehicle to remain on the roadway at the horizontal curve sections. TxDOT wanted to specifically compute 

and compare the margins of safety with respect to three safety improvements: high friction surface treatments (HFST), 

superelevation corrections, or curve realignment. 

FRICTION SUPPLY FRICTION DEMAND MARGIN OF SAFETY 

TCMS, shown in screenshot, is a spreadsheet tool that 

considers roadway and curve geometry, regulatory and 

predicted vehicle speed, traffic volumes, and other factors 

to estimate side friction demand at key points along a curve. 

By combining the side friction demand estimates with the 

knowledge of existing pavement friction values, the margin 

of safety is assessed for the point of curvature (PC), midpoint, 

and point of tangent (PT) by inputting the actual and 

proposed curve conditions. The proposed conditions may 

indicate a: 

TCMS Screenshot 
NOTEWORTHY PRACTICE
 

TEXAS	 HIGH FRICTION SURFACE TREATMENT (HFST) 

1. Variation in the pavement friction, indicating the 

application of HFST as a potential solution. 

2. Variation in superelevation, indicating 

superelevation corrections as a potential solution.	 

3. Variation in curve radii, indicating curve 

realignment as a potential solution. 

The resulting margin of safety allows users to understand 

which safety improvement is most likely to yield better 

results for the site. 

Additionally, TCMS allows the analyst to examine crash 

trends on a curve of interest to determine if the observed 

crash count is in line with what would be predicted for a 

similar curve. Occurrence of a higher-than-expected crash 

frequency can suggest that the curve needs to be treated. 

Key accomplishments, including roadway 

safety improvements:	 

This method can be used to prioritize proposed 

treatments at a set of curve sites, identify points along the 

curve where roadway departure crashes due to pavement 

friction deficiencies are most likely to occur, and evaluate 

several treatment alternatives besides HFST. In particular, 

the provision of enhanced pavement friction, increasing 

superelevation, and increasing curve radius through 

realignment are designed to increase the curve’s margin 

of safety, either by increasing side friction supply or by 

reducing side friction demand. 1 

The TCMS tool has allowed TxDOT to confidently assess 

horizontal curve safety within the State and increase HFST 

application with 125 installations planned. HFST is now 

included in TxDOT’s Highway Safety Improvement Program 

(HSIP). The TCMS tool also allows TxDOT to identify the 

critical points of a curve that need treatment by identifying 

the margin of safety at the PC, midpoint and PT. 

What technical and/or institutional changes 

resulted from the new practice? 

TCMS can be used to estimate the initial effectiveness 

of a proposed surface treatment plus its decrease in 

effectiveness over time if the analyst provides data about 

the life-cycle friction performance of the treatment. 

With the help of the tool, TxDOT was able to develop the 

following guidance to ensure that roadway user benefits 

and life-cycle costs are considered: 

• A proposed surface treatment must be determined 

for its sufficiency to meet the margin of safety 

requirements for a given curve before it is installed. 

• It must be determined whether, or how often, the 

surface treatment will need to be replaced so the 

needed margin of safety will continue to be available. 

This study also resulted in a guidance material, the TCMS 

spreadsheet, to help practitioners assess the need and the 

potential safety benefit of curve pavement improvements, 

including HFST. 

What benefits were realized as a result of 

the practice? 

The evaluation tool enabled agencies to foresee the 

benefits of HFST and other horizontal curve safety 

improvements before their application. Given the 

installation and maintenance costs of HFST, and the 

treatment not being recommended for pavements with 

limited structural integrity, it is crucial to optimize site 

selection and application of HFST. 

TxDOT is increasing its efforts to install friction treatments 

on curves, particularly through their HSIP. By using the 

TCMS tool, TxDOT will be able to target curves that stand 

to benefit the most from the application of a treatment, 

thereby making most efficient use of the limited safety 

improvement funds. TxDOT will also be able to identify the 

critical points of a curve that need treatment by identifying 

the margin of safety at the PC, midpoint and PT. 

 Michael Pratt et al. Evaluating the Need for Surface Treatments to Reduce Crash Frequency on Horizontal Curves. Report No. FHWA/TX-14/0-6714-1. May 2014. 

FOR MORE INFORMATION 

Joseph Cheung Darren McDaniel 

FHWA Office of Safety Texas Department of Transportation 

joseph.cheung@dot.gov darren.mcdaniel@txdot.gov http://safety.fhwa.dot.gov

FHWA-SA-16-059 

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