Accident mitigation on congested rural two-lane highways

Institute of Transportation Engineers. ITE Journal, Apr 2002 by Fitzpatrick, Kay, Harwood, Douglas W, Potts, Ingrid B

THE ACCIDENT MITIGATION GUIDE FOR CONGESTED RURAL TWO-LANE HIGHWAYS WAS DEVELOPED UNDER AN NCHRP PROJECT TO PROVIDE ONE COMPREHENSIVE DOCUMENT THAT A PRACTITIONER CAN USE TO INVESTIGATE SEVERAL POTENTIAL COUNTERMEASURES FOR IMPROVING SAFETY AND/OR OPERATIONS ON RURAL TWO-LANE HIGHWAYS.

THE MAJORITY OF THE U.S. highway system consists of two- and threelane rural roads, which represent four times the highway mileage of urban roads on the U.S. highway system. A substantial mileage of these roads carries relatively low traffic volumes and has few operational problems. Some of these rural highways, however, are becoming congested due to developing communities in exurban areas, heavy recreational travel, seasonal residences and special events. Increased congestion and travel demands can lead to an increased number of accidents.

Problems on rural highways have been related to three basic causes:

Inadequate road geometry (e.g., width, grades, alignment and sight distance) either at specific locations or over long sections;

Lack of passing opportunities, due either to limited sight distance or heavy oncoming traffic volume; and

Traffic conflicts due to turns at intersections and driveways.

These problems could be overcome with construction of a four-lane divided highway; however, such improvements are expensive, and the funds for upgrading rural roadways are often limited. As a result, constructing a new four-lane highway or widening an existing roadway may upgrade only a small portion of the network, while the other road sections continue to operate unsatisfactorily. As an alternative, lowercost improvements on existing two- and three-lane rural roads can solve many operational problems while improving a much larger portion of roadways. Research and experience have shown that the provision of passing lanes, turning lanes, localized alignment improvements, signs and pavement markings, median treatments, public information and education, increased enforcement and other relatively low-cost measures can be highly cost-effective in improving both traffic operations and safety on existing two- and three-lane rural roads. Since little information exists to help transportation practitioners evaluate the effectiveness of such countermeasures, a guide that presents safety-improvement options for congested rural and exurban two- and three-lane highways is needed.

RESEARCH OBJECTIVES AND SCOPE

The objectives for a recent National Cooperative Highway Research Program (NCHRP) project included the following:

Develop a guide that aids the transportation practitioner in selecting projects to improve safety on congested rural and exurban two- and three-lane highways;

Investigate examples of specific countermeasures that have been implemented by highway agencies in the past to alleviate safety problems on congested rural and exurban two- and three-lane roadways; and Determine the role of congestion in increasing accident rate and severity and in causing accidents of specific types through the statistical analysis of accident data for congested rural and exurban two- and three-lane roadways.

The development of a guide (called the Accident Mitigation Guide for Congested Rural Two-Lane Highways1) satisfied the first two objectives for the NCHRP project. It included information on specific roadwayimprovement projects along with information on accident countermeasures. (Some of the projects were also discussed in an ITE 1999 Spring Conference and Exhibit paper.2) The findings from the third objective are provided in the (unpublished) final report3 and are summarized in this feature.

ACCIDENT MITIGATION GUIDE

While the project had several tasks and objectives, its primary purpose was the creation of the Accident Mitigation Guide for Congested Rural Two-Lane Highways. The Accident Mitigation Guide was to be developed to provide assistance to the transportation practitioner in identifying and designing projects to improve safety on congested rural and exurban two- and three-lane highways. A synopsis of the material in the Accident Mitigation Guide is provided in Table 1.

Chapters 3 to 6 of the Accident Mitigation Guide contain the bulk of information.

They discuss countermeasures that are appropriate for congested rural and exurban two- and three-lane highways. Each countermeasure section starts with an overview, such as a brief discussion on the need for adequate recovery distance along a roadway. This discussion is then followed by three subsections: Accident Experience, Countermeasures and Effectiveness of Countermeasure. Accident Experience contains available information on the types of accidents and/or the frequency of accidents for the situation. The next discussion on appropriate countermeasures for use presents general information about the countermeasure, techniques that are used and examples (see Figure 1 for an example of a rumble strip). The final subsection discusses the known effectiveness of the countermeasures. In some cases, the effectiveness of a countermeasure is well known, such as the addition of shoulders. In other cases, the effectiveness is suspected or not known.