Objective criteria for guardrail installation.

The establishment of a more objective basis for warrants of guardrail placement on embankments and adjacent to freeway fixed objectives is studied. A mathematical model was developed to compare the relative safety at any embankment or fixed object location with the relative safety of protective guardrail at a similar location. At any location the relative safety is dependent on two variables---accident severity and accident frequency. If two similar locations, one with guardrail and one without, have the same accident frequency, the location with the lower accident severity is safer. The relationship developed to evaluate accident severity was named the severity index and was a weighted average of the accident severity values for a given condition using the ratios of direct costs of single vehicle accidents by severity class; fatal, injury, and property-damage-only (pdo). The ratios are 25, 6, and 1 for the fatal, injury and pdo accidents, respectively. The relationship developed to evaluate accident frequency was named the probability index and is simply the number of accidents per vehicles exposed to the condition. The true comparison measure, the product of the severity index and probability index, was called the collision index. Also a field inventory of fixed objects and fixed object guardrail was made on 95% of the 1963 freeway mileage to determine the relative exposures to each fixed object and gaurdrail type. The following conclusions were arrived at based on a collision index analysis: (1) guardrail placed adjacent to bridge rail ends, abutments, and piers decreases the collision index (improves safety). (2) guardrail placed adjacent to steel signposts also decreases the collision index (improves safety). (3) guardrail placed adjacent to lightpoles increases the collision index (decreases safety). (4) steel signposts placed in the off-ramp gore areas are considerably less safe than steel signposts adjacent to the highway shoulder. And (5) the use of dimensional lumber signposts in lieu of steel signposts reduces the accident severity.