This study investigated many important issues associate with pavement surface friction testing, in particular using the smooth tire. This study utilized 3-D FEM program to investigate the fundamental friction phenomenon in light of energy dissipation during friction process. It was demonstrated that the pavement friction depends on many factors such as test tire, test speed, surrounding conditions, pavement surface texture, and pavement type. A great amount of friction data has been collected so as to investigate variations involved in pavement friction measurements. System variations depend on the feature of pavement surface. The standard deviations due to system errors are usually less than 5. The smooth tire tends to provide greater variations than the ribbed tire. As air temperature increases, the friction number does not necessarily decrease. No consistent relations were identified between friction measurements and test seasons. Seasonal friction variations are negligible. The largest directional variation is 16 with the smooth tire on a State road. The State and U.S. roads tend to produce greater directional variations than the interstates. Driving lane usually has lower friction than other lanes. The greatest lateral variation arose due to the effect of wheel track. Longitudinal friction variations depend on traffic distribution, pavement type, and surrounding conditions. Friction measurements taken at 1.0-mile spacing can provide realistic network pavement friction information. Pavement frictions on interstates decreased faster than those on State and US roads. INDOT conducts pavement inventory friction test every year on interstates and every three years on State and US roads. The force transducers should be calibrated every month and the whole system performance verified every week so as to identify potential significant performance changes. A minimum of three to five test runs must be conducted for system verification. The standard smooth tire is recommended for INDOT network pavement inventory friction test. In general, the friction number measured with the ribbed tire is greater than that with the smooth tire. However, the differences decrease as the surface texture becomes rougher. The average friction difference is about 20 on highway pavements. Friction test speed should be determined in light of the traffic conditions. Three test speeds of 30 mph, 40 mph, and 50 mph are recommended for network pavement inventory friction testing. Determination of the minimum friction requirement should consider its impact on wet-pavement accidents and agency’s budgets. Taking into account the minimum friction requirement recommended by NCHRP Report-37 and the differences between the ribbed and smooth tires, a friction number of 20 with the smooth tire at 40 mph is recommended as the minimum friction requirement for network pavement inventory friction testing. It was found that this requirement is economically reasonable in light of the network pavement maintenance and resurfacing. No good correlations were identified between pavement friction and wet-pavement accidents.

Report Number



Pavement friction, tire-pavement interaction, energy dissipation, 3-D simulation, smooth tire, frictional variations, friction requirement, network pavement, SPR-2821

SPR Number


Project Number


File Number


Performing Organization

Joint Transportation Research Program

Publisher Place

West Lafayette, IN

Date of this Version