Traffic: traffic volume, heavy duty vehicles, variation of traffic flow, dangerous goods transports, posted speed, real speed, variation of speed, enforcement of traffic regulations

Road: uniform vs. varying environment, merging traffic, intersections, longitudinal gradient, cross cradient, curvature, line of sight, number of traffic lanes, width of lanes, road marking, separation of traffic directions, deterioration of road surface

Vehicles: vehicle age, regular vehicle check-up (or not), average and variation of vehicle weight

Drivers: training, education, DUI, discipline, fatigue

Environment: light, wind, rain, ice/snow

I am sure, the list is not complete.