Unsignalized Intersection Collisions

Type of Problem Being Addressed

General Description of the Problem

Intersections constitute only a small part of the overall highway system, yet intersection-related crashes constitute more than 50 percent of all crashes in urban areas and over 30 percent in rural areas (Kuciemba and Cirillo, 1992). Fatal intersection crashes are a smaller portion of the total picture, suggesting that severity of crashes at intersections is lower than elsewhere (Exhibit III-1).

Exhibit III-2 shows the severity of crashes at unsignalized intersections as estimated for the United States in 1999 by the National Center for Statistics and Analysis (NCSA) General Estimates System (GES), operated by NHTSA.

EXHIBIT III-1 Fatal Crashes by Location

EXHIBIT III-2 Severity of Accidents at Unsignalized Intersections

It is not unusual that crashes are concentrated at intersections, because intersections are the point on the roadway system where traffic movements most frequently conflict with one another. Good geometric design combined with good traffic control can result in an efficient and safe intersection.

A recent analysis of California data found that, on average, 1.5 crashes per year occur at unsignalized intersections in rural areas, compared with 2.5 crashes per year in urban areas (Bauer and Harwood, 1996). By contrast, urban signalized intersections averaged 4.6 crashes per year. However, these values are average—many intersections have substantially higher crash frequencies, and these higher frequencies are the appropriate targets for improvements. There are many more unsignalized intersections than signalized, so the number of crashes is undoubtedly much higher at unsignalized intersections nationwide than at signalized intersections.

As population and development increases, traffic at unsignalized intersections grows, as does traffic volume and the number of crashes. There is increasing demand for signalization of urban and suburban intersections, and, even in rural areas, signalized intersections are becoming more common. However, experience shows that intersection crash rates frequently increase with signal installation, although the crashes may be less severe. Signalization usually leads to a shift in crash types, with fewer angle and turning collisions and more rear-end collisions.

Specific Attributes of the Problem

Exhibit III-3 depicts the distribution of fatal crashes by manner of collision. It shows the preponderance of angle crashes, and the significant role of crashes with fixed objects. Angle crashes include a large proportion of turning vehicles, especially left turns.

Other specific attributes of safety problems at unsignalized intersections include

• Diverse geometric design and traffic control features of intersections;
• Diverse vehicle population, including a wide range of vehicle sizes and weights;
• Diverse driver populations, including anticipated demographic changes;
• Climate-related problems, such as wet and ice-and-snow-covered pavements; and
• Wide range of traffic volumes on intersection approaches, with patterns of daily and seasonal variations.

Of these attributes, only geometric design and traffic control features are under the direct control of highway agencies. Aspects of problems related to other driver and vehicle issues are addressed in other parts of the AASHTO SHSP that are not specific to unsignalized intersections.

EXHIBIT III-3 Manner of Collision for Fatal Crashes at Unsignalized Intersections

EXHIBIT III-4 1999 Unsignalized Intersection Accidents First Harmful Event