Extensive single-wheel traction tests were conducted in the vicinity of UC Davis campus using four different radial ply tires, two soil types, four soil conditions, three levels of inflation pressures, and three axle loads. Soil sinkage and shear characteristics were also obtained using an instrumented soil test device in every test condition. These field data were analyzed to obtain semi-empirical traction prediction equations for radial ply tires. In general, these prediction equations were able to predict traction equation parameters with less than 25% error.