This report presents the research effort to explore the use of continuous friction measurement equipment (CFME) as a tool for pavement friction management to be incorporated into the North Carolina Department of Transportation (NCDOT) Pavement Management Program to produce a stronger Safety Improvement Program. CFME data could supplement and/or replace the existing locked-wheel tangential friction measurements and provide critical details to better understand road departures, wet crashes, and overall traffic performance and safety along ramps, loops, curves, and super-elevated sections where assessing friction using traditional methods is difficult. The more-detailed data that result from such efforts could help to better identify the most appropriate and effective treatments and better define the limits of the needed treatment.
Testing included measurements with the locked-wheel trailer currently used by NCDOT, a Grip Tester, and a Side-Force Coefficient Routine Investigation Machine (SCRIM). The researchers took measurements with the three devices on common pavement types, overlays, and surface treatments on various highways in the state. The results from the three different machines and methodologies were compared, and guidance for future implementation is provided.
The research products include:
- A comparison of friction obtained from the three different machines and methodologies, including continuous average friction values by pavement type for all the roadway geometries tested (curve/ramp/loop/super-elevated section/section on grade).
- Recommendation and guidance with regard to the feasibility of collecting continuous friction and macrotexture data to define investigatory friction and macrotexture levels to support the state's pavement friction management program.
The main conclusions of the review of practice and analysis of the data collected as part of the study are the following:
- The direct results of the comparison showed that it is possible to interconvert Grip Number (GN) and SCRIM Reading (SR) measurements with locked-wheel skid tester (LWST) measurements but the correlations are not very strong. This is consistent with the results of several reviewed international efforts.
- Macrotexture is a very important parameter for understanding the pavement's full frictional properties, especially for those devices that are insensitive to it (such as the LWST with a ribbed tire). There is significant consensus on the impact of macrotexture on total crashes.
- The development and implementation of a Pavement Friction Management program would benefit from the collection of continuous friction and macrotexture data. This can facilitate the definition of investigatory friction levels that can be used to flag sections with marginal friction levels based on crash trends. In addition, the cost of data collection per mile is lower than the traditional approach and provides a better characterization of the pavement frictional properties.
Based on the stated conclusions the following recommendations are provided:
- NCDOT should investigate the feasibility of collecting macrotexture data to complement the agency's friction data collection. This will allow areas with potentially deficient macrotexture to be identified and investigated at the project level and corrected, if necessary, before the occurrence of wet-weather crashes.
- NCDOT should also investigate the feasibility of implementing a proactive friction management program that uses a CFME with macrotexture measurement capabilities to define threshold investigatory levels and use safety performance functions (SPFs) to identify sites with the highest potential payoff for friction improvement.
