Roadway Departure Safety: A Manual for Local Rural Road Owners

4. Countermeasures

The decision regarding which countermeasures to install to address a safety issue can be challenging. When necessary, the local practitioner should seek engineering expertise from a county engineer, State DOT, or through the State Local Technical Assistance Program (LTAP). To make the most informed decision regarding roadway departure countermeasure selection, an agency should use all available data, both quantitative and qualitative. Through the work of several agencies (Federal, State and local) and universities, several proven, effective countermeasures have been identified to address roadway departure crashes. When using the countermeasure-based systematic approach, the practitioner should determine the appropriate conditions for which the countermeasure is most effective (crash type, geometric features, traffic conditions).

For a relatively small number of high crash locations with varying causes, the spot location implementation approach may be the most appropriate. Systematic implementation of proven low-cost safety countermeasures is often the most effective approach when there are several locations on the roadway network experiencing similar types of roadway departure crashes. For locations that have yet to experience crashes, but have identified features that contribute to roadway departure crashes, systematically applying safety treatments may be the best approach to prevent future crashes.

A high proportion of crashes tend to occur at locations that share common geometric or operational elements. Installing the same countermeasure at multiple locations (where appropriate) could, in many cases, increase the cost effectiveness of the safety improvement, allowing an increased number of treatments to be applied.

4.1 Select Roadway Departure Countermeasure

The countermeasures discussed in this section of the document are not all-inclusive of all those available to reduce the frequency and severity of roadway departure crashes on local, rural roads. Those discussed have shown to be effective for the appropriate roadway departure crashes.

The determination of sites to consider for countermeasure implementation is based on the analysis performed. Each countermeasure discussed in this section includes the following information:

• Crash Type Addressed – In order to effectively reduce the number and severity of roadway departure crashes, it is necessary to match countermeasures to the crash types they are intended to address. Depending on the type of problem, one or more of a wide range of treatments could be the most effective way to reduce the number and severity of future crashes.
• For example, if the most frequent crash type on a corridor involved vehicles running off the road around curves, the most effective countermeasures will likely be those that address curve crashes directly, which include curve warning signs, curve alignment signs, and pavement marking.
• Where to use – Roadway segments with specific common characteristics can be addressed with similar countermeasures that are most effective.
• Why it works – A discussion of the benefit of a countermeasure is important to determine its appropriateness in addressing certain roadway departure crash types at areas with specific issues as determined by the data and roadway features.
• Timeline for Implementation – This category refers to the approximate relative time it can take to implement the countermeasure.

• Estimated cost – Most countermeasures included in this report are considered low cost; low to moderate cost; or moderate cost. These categories represent relative costs of the countermeasures to each other. Note that costs can vary considerably due to local conditions.
• Crash Reduction Factor – The crash reduction factor (CRF)⁹ is an indication of the effectiveness of a particular treatment, measured by the percentage of crashes it is expected to reduce. The CRF for a given countermeasure is calculated based on research conducted on the pre- and post-crash frequencies at sites where the countermeasure has been installed. The higher the CRF, the greater the expected reduction in crashes. For instance, a CRF of 20 is interpreted as an expected reduction of 20 percent of previously-experienced crashes. The effect of a countermeasure on crashes can also be expressed as a Crash Modification Factor (CMF). It is defined mathematically as 1 – CRF. In the example above, a 20 percent reduction of crashes is represented by a CMF of 0.80. This effect of the countermeasure can be calculated by multiplying its value by the number of current crashes to determine an expected number of future crashes. For instance, if there are 50 current crashes, by multiplying the CMF of 0.80 times 50, one could expect 40 future crashes.

⁹ Federal Highway Administration in conjunction with the University of North Carolina Highway Safety Research Center, "Crash Modification Factors Clearinghouse" website. Available at: http://www.cmfclearinghouse.org/.