Introduction: Roadway departure crashes, including those involving traffic barriers such as bridge rails and guardrails, tend to be frequent and severe in nature, specifically for roadways serving high traffic volumes at high speeds. In 2013, the National Highway Traffic Safety Administration (NHTSA) observed that, in the United States, collisions with fixed objects and non-collisions incidents account for only 18 percent of all reported crashes; however they result in 44 percent of all fatal crashes.

Methods: That said, this paper explored significant relationships between roadway elements, its surroundings, design, and condition characteristics using statistical analysis and regression modeling to better understand associative properties of roadway and bridge characteristics on the frequency and severity of crashes involving bridges and guardrails.

Results: The (crash) frequency results revealed that vehicular crashes involving bridges are very rare events. In the 10-year analysis period from 2004 to 2013, there were merely 862 single-vehicle bridge crashes reported in Iowa. Nonetheless, crashes involving bridges are more frequent on some bridges more than others. In conjunction with previous studies, bridges characterized to have increased traffic volumes and lengths are more susceptible to crashes. In general, bridges designed to old design standards or meeting substandard (superstructure, railing, or alignment) conditions are also more susceptible to crashes over time. Conversely, bridges characterized to have relatively wider widths than their travel ways, and bridges on paved roadways are less susceptible to crashes.

The (occupant injury) severity results revealed that collisions involving bridges tend to be more severe for unprotected vehicle occupants. Also, concurrent with past studies, driving under the influence of drug or alcohol increases the probability of more severe injuries. On the contrary, bridge crashes during wet, icy, snowy, or slushy road surface conditions however tend to be less severe.

Practical Applications: While the low quantities of bridge crashes on county roads may be indicative of bridge rails and guardrails serving their purpose, the findings of this study can be useful to local public agencies regarding guidance for bridge and barrier rail upgrade standards. Special consideration/emphasis may be placed on bridges possessing certain characteristics because they expect higher crashes, although a relatively small proportion of bridges may actually possess these characteristics.

That said, prescriptive guidelines for bridge rail use on county roads may not be necessary, given the rareness and randomness of crash events. However, other effective approaches may include the use of categorical thresholds for establishing practical requirements of when and how county engineers should upgrade bridges.