Hummer et al. (2010) looked at the characteristics of collisions occurring on two-lane curves, by analysing the crash data from the North Carolina Highway Safety Information System. Including collisions with fixed objects and rollovers as ROR type collisions, the study found that 60% of crashes occurring on two-lane curves in urban environments were ROR type collisions. In contrast, ROR type crashes amount to 33% of total collisions occurring at either straight or curved portions of all two-lane roads (urban and rural), and 10% of collisions occurring on any urban road. This suggests that both the curvature and width of the road were contributory factors.