This investigation evaluated when guardrail placed adjacent to a freeway fixed object or along the top of an embankment is safer for the occupants of a colliding vehicle than the "unprotected" object or the embankment. This was done by determining when the Collision Index, a mathematical Measure of the combined effect of accident severity and accident frequency, is less for guardrail than for a particular type of fixed object or a particular combination of embankment height and slope. The freeway fixed objects studied were bridge-rail ends, bridge abutments and piers, light poles, steel signposts adjacent to the highway shoulder, steel signposts in the off-ramp gore area, and wood signposts. The Collision Index for each of these types of fixed object was determined with and without guardrail protection.
Abstract