Safety and Cost-Effectiveness of Approach Guardrail for Bridges on Low-Volume Roads

The bridge approach guardrail is a commonly used safety feature designed to prevent collisions with bridge components and other types of runoff-the-road crashes occurring on the approach or departure to bridges. The average daily traffic (ADT) threshold at which an installation of bridge approach guardrail on low-volume roads is cost-effective was determined on the basis of reductions in crash severity. Logistic regression and chi-squared tests were used to analyze characteristics of 96 run-offthe-road crashes that occurred on the approach or departure to 68 county state-aid highway bridges in 10 Minnesota counties over a 15-year period. Crashes that occurred at bridges with approach guardrail were found to be significantly less severe than crashes that occurred at bridges without guardrails. None of the 33 crashes with approach guardrail resulted in a fatality or severe injury, whereas roughly one-quarter of the 63 crashes with a roadside or bridge rail end resulted in a fatality or severe injury. Crashes with the approach guardrail were much more likely to result in property damage only compared with crashes with the roadside or bridge rail end. Bridge approach guardrail installed at all four quadrants of a bridge has a benefit-cost ratio ranging from 3.99 to 6.62 and is cost-effective at ADTs greater than or equal to 400. An ADT threshold of 400 is recommended for installation of a bridge approach guardrail on low-volume roads; this is consistent with current roadside clear zone guidelines suggested by AASHTO for very low-volume local roads.