An extensive review of the literature indicates a far greater focus on ROR collisions, particularly in an urban road environment. This may be attributable to the greater incidence and severity of ROR collisions in this environment. However, as Larsson et al. (2003) highlight, head-on collisions can frequently be considered as a sub-category of ROR collisions, in which the errant vehicle collides with a vehicle travelling in the opposite direction, rather than a roadside object. Thus, head-on collisions will frequently share the same contributory factors, and crash reduction treatments will frequently be similar.