Horizontal curves

Type of Problem Being Addressed

General Description of the Problem

Statistics from the Fatality Analysis Reporting System (FARS) indicate that 42,815 people were killed in 38,309 fatal crashes on the U.S. highway system in 2002. Approximately 25 percent of these fatalities occurred along horizontal curves (see Exhibit III-1). Curves are necessary and important elements of every highway. Considering these statistics and that the average accident rate for horizontal curves is about three times the average accident rate for highway tangents (Glennon et al., 1985), implementing strategies designed to improve the safety of horizontal curves will help achieve the overall goal of the AASHTO Strategic Highway Safety Plan.

 

Specific Attributes of the Problem

Accidents on horizontal curves cause a significant amount of pain and suffering to those involved in the accidents because of the nature of the collisions. For example, while only slightly less than 2 percent of all crashes on curved roadway segments are fatal crashes, approximately 40 percent involve some type of injury (see Exhibit III-2). Many of the more severe curve-related crashes (i.e., fatal) occur primarily in rural settings. Exhibit III-3 shows that about three-quarters of the fatal crashes occur in rural areas. Exhibit III-4 shows that more than 70 percent of the fatal crashes occur on secondary roads, which implies that many of these are on roads that are not under state DOT jurisdiction. Finally, Exhibit III-5 indicates that attention to the problem of crashes on curves should be directed primarily to two-lane highways in rural areas.

 

The safety of curves is a reflection of both the roadway itself and the roadside environment. Exhibit III-6, which includes crashes of all severities along curved roadway segments (GES, 1999), shows that the first harmful event of a crash on a curved highway segment is just as likely to occur on the traveled way as off the traveled way. (The “Other” category includes locations such as within interchanges.)

The most prevalent types of crashes that occur at horizontal curves are run-off-road (ROR) and head-on crashes. Exhibit III-7 shows that 76 percent of curve-related fatal crashes were single-vehicle crashes in which the vehicle left the roadway and struck a fixed object or overturned, while 11 percent of curve-related fatal crashes were head-on crashes.

 

Thus, ROR and head-on crashes accounted for 87 percent of the fatal crashes at horizontal curves. Accordingly, the strategies for improving safety at horizontal curves focus on reducing the frequency and severity of these specific types of crashes.