EFFECT OF PRECEDING TANGENT LENGTH ON SAFETY FOR HORIZONTAL CURVES

Most safety researchers have long supposed that sharp horizontal curves are likely to have higher accident rates when preceded by long tangent sections of roadway than when preceded by short tangents or another horizontal curve. It appears reasonable that a long tangent might decrease a driver's vigilance and that an encounter with a sharp curve at the end of the tangent might surprise the driver, resulting in a higher accident risk. Many studies have remarked on this phenomenon, but there has been little hard evidence. A recent paper by Hauer (1999) points to a New Zealand study by Matthews and Barnes (1988) as providing the most credible empirical evidence of this effect. Matthews and Barnes found that the accident rate of a horizontal curve with radius less than 500 m increases as the preceding tangent length increases, up to a preceding tangent length of 1200 m. Other investigators who have addressed the issue include Datta et al. (1983), Terhune and Parker (1986), Zegeer et al. (1990), Brenac (1996), and Fink and Krammes (1995), as well as a British study (Road Research Laboratory, 1965) and a French study. This paper uses data from the U.S. Federal Highway Administration (FHWA) Highway Safety Information System (HSIS) on the safety performance of horizontal curves and their preceding tangents on rural two-lane highways to reexamine this issue. The HSIS data for the State of Washington includes data on the location and geometric design features of each individual horizontal curve and tangent section. These geometric data have been linked to an accident database containing three years of records on individual accidents. A key strength of the database for this analysis is that it includes all rural two-lane highways under State jurisdiction, not just a sample. The paper examines whether the accident rate of a horizontal curve increases with preceding tangent length and how that effect varies with horizontal curve radius. The results have been cast in a form that can be used in the accident prediction algorithm for rural two-lane highways being developed for the FHWA Interactive Highway Safety Design Model (IHSDM).