Updating Side Friction Factors for Design of Unpaved Roads

Traction coefficients are used in the calculations required for safe geometric road design, including stopping sight distance and horizontal curve alignment. Friction factors are based on traction coefficients for low-volume roads provided in the U.S. Department of Agriculture Forest Service (FS) Road Preconstruction Handbook and are also referenced by the AASHTO Guidelines for Geometric Design of Very Low-Volume Local Roads (ADT ≤ 400). These traction values are based on measurements from the 1950s to the 1970s. However, tire design, construction, and even test methods have changed considerably since then. To evaluate the validity of the design coefficients, and to determine replacement values or ranges, a literature search, data mining, and a limited test program were conducted by the U.S. Army Cold Regions Research and Engineering Laboratory and FS. This paper presents existing FS handbook values and design guidelines, updated values collected from the literature, and new measurements taken on gravel and snow-covered roads. For longitudinal friction–traction, updated values are similar to the FS handbook values, except for asphalt and gravel, which has presented friction values up to 30% and 100% higher, respectively. Lateral traction shows even higher values for gravel and snow, being 2 to 2.5 times as high as the handbook's median values. Fortunately, these differences err on the side of safety. Future evaluations should consider the impact of the higher values on road design, operations, and economics to see whether a change is warranted. Testing of additional surfaces is recommended, as are measurements that use a wider range of tires and vehicles.