Signalized Intersections

Type of Problem Being Addressed

General Description of the Problem

Intersections constitute only a small part of the overall highway system, yet intersectionrelated crashes constitute more than 20 percent of fatal crashes. 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 intersection that operates efficiently and safely.

Exhibit III-1 shows the breakdown of fatal crashes by facility type, which is referred to as “relation to junction” in the Fatality Analysis Reporting System (FARS) database. Just under a quarter of fatal crashes occur at intersections.

EXHIBIT III-1
Fatal Crashes by Relationship to Junction
Source: 2002 FARS data. “Other” includes crashes categorized in FARS as related to railroad grade crossings, crossovers, and unknown.

A brief analysis of FARS data for 2002 shows the following:

  • 23 percent of all fatal crashes occurred at intersections,

  • 6 percent of all fatal crashes occurred at signalized intersections,

  • 29 percent of fatal crashes at intersections occurred at signalized intersections,

  • 84 percent of fatal crashes at signalized intersections occurred in urban areas, and

  • 59 percent of fatal crashes at signalized intersections involve angle collisions with other vehicles.

Exhibit III-2 shows the distribution of fatal crashes at signalized intersections by manner of collision. The high percentage of crashes that do not include a collision with another moving vehicle can be attributed to pedestrian and bicycle crashes. FARS data show that 75 percent of the fatal single-vehicle crashes at signalized intersections involve pedestrians or bicyclists (55 percent of fatal single-vehicle crashes at all intersections involve pedestrians or bicyclists).

EXHIBIT III-2
Manner of Collision for Fatal Crashes at Signalized Intersections
Source: 2002 FARS data. “Other” includes crashes categorized in FARS as sideswipe same direction, sideswipe opposite direction, other, and unknown.

Exhibit III-3 shows the distribution of severity of crashes at signalized intersections in the United States, compared to stop-controlled intersections and nonjunctions along which there is no signal control. While nonjunctions proportionately experience slightly more fatal crashes, proportionately fewer injury crashes are associated with them than intersection crashes. There is little difference between the distributions for the two types of intersection control.

Analysis of the crash types at a signalized intersection helps focus the efforts for implementing improvements. The descriptions of strategies in this guide discuss the crash types affected by the strategies. The focus of this guide is on reducing fatalities at signalized intersections through low-cost, short-term improvements. The approach is to provide comprehensive strategies that include intersection design features (e.g., sight distance, leftand right-turn lane presence and design, skew angle, number of legs), as well as traffic operational factors (e.g., number of phases, type of signal phasing, timing, and signal progression), enforcement factors such as red-light running, and improved emergency response measures such as signal preemption.

EXHIBIT III-3
Comparison of Severity Distribution of Crashes
Source: NHTSA, FARS , General Estimates System (GES) 2002.

Fifty percent of all crashes in urban areas and over 30 percent in rural areas (Kuciemba and Cirillo, 1992) are intersection related. For signalized intersections, 85 percent of fatal crashes occur in urban areas (see Exhibit III-4). Since most traffic signals are located in urban areas, this breakdown by area type makes sense. Many of the strategies discussed in this guide are more feasible in urban situations than in rural ones; this is appropriate due to the prevalence of fatal crashes at urban signalized intersections.

EXHIBIT III-4
Fatal Signalized Intersection Crashes by Rural and Urban Areas
Source: 2002 FARS data.