Road departure crashes are often severe and account for the majority of highway fatalities nationwide. Road departures account for 67 percent of fatal crashes in Iowa.
The Federal Highway Administration (FHWA) defines a roadway departure crash as "a non-intersection crash [that] occurs after a vehicle crosses an edgeline or a centerline, or otherwise leaves the traveled way" (FHWA 2015).
Roadway departures are also problematic since they frequently occur in rural areas. Emergency medical service response times in rural areas are 1.6 to 2 times longer than for urban areas (Gonzalez et al. 2009, NHTSA 2006) and fatal injury crash rates are 2 to 3 times higher in rural than urban areas (NHTSA 2005, Zwerling et al. 2005). Pedestrians are more than twice as likely to be killed in a pedestrian-vehicle collision in a rural area than in an urban area (Mueller et al. 1988).
Several studies have been conducted in Iowa to address lane departure crashes. A number of other studies were conducted by other groups and are relevant to Iowa as determined by a technical advisory committee (TAC).
A list of general resources developed in Iowa to reduce lane departure crashes is found here.
A summary of known performance for each lane departure countermeasure is provided in the table below.
|Lane Departure Countermeasures||Highest Speed Impact (mph)||CMF1||Other Crash Impacts1||Cost2||Usage Within Iowa||Iowa-Specific Guidance|
|chevrons||-1.6||0.75 to 0.96||N/A||$||common||refer to MUTCD|
|chevrons/warning signs||N/A||0.56||N/A||$||common||refer to MUTCD|
|on-pavement curve sign||-7.0||N/A||N/A||$||limited|
|post mounted delineators||-8.0||0.70 to 0.80||N/A||$||limited||refer to MUTCD|
|conspicuity for chevron posts||-2.2||N/A||N/A||$||limited|
|dynamic speed feedback signs|
|DSFS on curves||-7.9||0.93 to 0.95||N/A||$$||limited|
|longitudinal rumble strips|
|centerline rumble strips||N/A||0.67 to 0.91||-25 to -37%||$$$||occasional|
|shoulder rumble strips||N/A||0.79 to 0.90||-8 to -70%||$$$||common|
|edgeline rumble stripes||N/A||0.50 to 1.30||N/A||$$$||tested|
|paved shoulder||N/A||0.81||-8%||$$$||widespread||Section 3D-6 Iowa Design manual|
|Safety Edge||N/A||0.94||$$||occasional||Section 3D-6 Iowa Design manual|
|high friction treatment||N/A||N/A||-20 to -25%||$$||limited|
|wider edge lines||N/A||0.38||-27%||$||no experience|
(1) Actual crash reductions and CMF will vary by crash type. CMF and crash reduction values which were determined to be the most appropriate for Iowa rural two-lane roadways were selected from a range of studies.
(2) Estimated installation costs ($ < $5,000; $$ $5,000 to $10,000; $$$ >$10,000) — actual cost depends on size of treatment location, actual product selected, whether treatment is done in conjunction with other activities such as resurfacing/paving, etc. Maintenance costs can vary considerably but are not estimated.
Federal Highway Administration. Roadway Departure Safety. Last accessed March 2015.
Gonzalez, Richard P., Glenn R. Cummings, Herbert A. Phelan, Madhuri S. Mulekar, and Charles B. Rodning. "Does Increased Emergency Medical Services Prehospital Time Affect Patient Mortality in Rural Motor Vehicle Crashes? A Statewide Analysis." The American Journal of Surgery 197 (2009): 30-34.
Mueller, B. A., F. P. Rivara, and A. B. Bergman. "Urban-Rural Location and the Risk of Dying in a Pedestrian-Vehicle Collision." Journal of Trauma 28 (1988): 91-94.
National Highway Traffic Safety Administration (NHTSA). Contrasting Rural and Urban Fatal Crashes 1994-2003. Report DOT-HS-809-896, 2005.
National Highway Traffic Safety Administration (NHTSA). Traffic Crashes Take their Toll on America's Rural Roads. Report DOT-HS-810-658, 2006.
Zwerling, C., C. Peek-Asa, P. S. Whitten, S. W. Choi, N. L. Sprince, and M. P. Jones. "Fatal Motor Vehicle Crashes in Rural and Urban Areas: Decomposing Rates into Contributing Factors." Injury Prevention 11 (2005): 24-28.
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