This paper describes an examination of the relationship between vehicular speed and side friction demand on horizontal curves. Models of speed and friction demand are described. The curve speed model explains the effect of approach speed, radius, and superelevation rate on curve speed. The side friction model explains the effect of approach speed and curve speed reduction on side friction demand. The terms in the side friction model reflect a general desire by motorists for a lower side friction demand at higher speeds. Model terms also reflect a willingness by motorists to tolerate slightly higher side friction demand in an effort to minimize the amount of speed reduction required by relatively sharp curvature. This model is recommended as a rational basis for defining the maximum side friction factors for use in curve design.