Requirements of a safety zone width for run-off-road accidents to prevent severe injuries when impacting hazardous objects

Single vehicle accidents (SVA), in most cases run-off-road (ROR) accidents are identified as a major burden on Europe’s roads and bear a particularly high risk. On average SVA fatalities account for one third of annual road accident fatalities. The injury severity in these accidents is dependent on the interaction between the vehicle and the roadside objects. The most frequent unshielded objects at the roadside are identified as trees and utility poles.
Roadside safety addresses the surroundings outside of the carriageway. A forgiving roadside could reduce the consequences of an impact. Ideally, the roadside is flat and is unobstructed by objects. This roadside, referred to as the safety zone or clear zone, is defined as the roadside border from the edge of the carriageway and can be used by errant vehicles to recover. The objective of the study is to analyse ROR accidents and define a safety zone width comprising of theoretical assumptions and real accidents. The lateral distance of the safety zone must accommodate the motions of the vehicles until a critical impact speed is reached. A critical impact speed was defined based on consumer regulations or real accident analysis.
Most of the unshielded objects (75%) in this study were found to be in a lateral distance of four meters to the edge of the carriageway. Unshielded narrow objects such as trees, utility poles, etc. were most frequent (48%). The average collision speed is 70.9 km/h (SD=23.8) and the median collision speed is 69 km/h. No clear indication between collision speed and lateral distance of objects to the edge of the carriageway has been identified. Even if only few collisions occur with objects at a great distance from the edge of the carriageway, the collision speed can nevertheless be up to 100 km/h.
The defining of a safety zone is suggested, which is dependent on the ROR speed. At a ROR speed of 80 km/h and a given allowed collision speed of 32 km/h, a safety zone width of 12.5 meters would be recommended. At a ROR speed of 100 km/h a safety zone width of 16.7 meters would be recommended and at 130 km/h the safety zone width would be 22.8 meters. Logically, the safety zone width would decrease with increased collision speed allowance.