Unsignalized Intersection Collisions

Executive Summary

Introduction

One of the hallmarks of the American Association of State Highway and Transportation Officials (AASHTO) Strategic Highway Safety Plan (SHSP) is to comprehensively approach safety problems. The range of strategies available in the guides will ultimately cover various aspects of the road user, the highway, the vehicle, the environment, and the management system. The guides strongly encourage the user to develop a program to tackle a particular emphasis area from each of these perspectives in a coordinated manner. To facilitate this coordination, hypertext linkages are provided on AASHTO's Web site at http://safety.transportation.org to allow for seamless integration of various approaches to a given problem. As more guides are developed for other emphasis areas, the extent and usefulness of this form of implementation will become more apparent.

The goal is to move away from independent activities of engineers, law enforcement, educators, judges, and other highway safety specialists. The implementation process outlined in the guides promotes forming working groups and alliances that represent all of the elements of the safety system. In this formation, highway safety specialists can use their combined expertise to reach the bottom-line goal of targeted reduction of crashes and fatalities associated with a particular emphasis area.

The six major areas of the AASHTO SHSP (Drivers, Vehicles, Special Users, Highways, Emergency Medical Services, and Management) are subdivided into 22 goals, or key emphasis areas, that impact highway safety. One of these goals addresses the improvement of safety at intersections. A key to improving intersection safety is to address safety problems at unsignalized intersections. This implementation guide provides guidance to highway agencies that want to implement safety improvements at unsignalized intersections and includes a variety of strategies that may be applicable to particular locations.

Intersections are locations where two or more roads join or cross one another. The crossing and turning maneuvers that occur at intersections create opportunities for vehicle-vehicle, vehicle-pedestrian, and vehicle-bicycle conflicts, which may result in traffic crashes. Thus, intersections are likely points for concentrations of traffic crashes. Unsignalized intersections are of particular concern because there are so many of them on the U.S. highway system and because some experience sufficient numbers of particular crash types to indicate a need to improve safety.

Unsignalized intersections represent potential hazards not present at signalized intersections because of the priority of movement on the main road. Vehicles stopping or slowing to turn create speed differentials between vehicles traveling in the same direction. This is particularly problematic on two-lane highways. The intersections along low- to moderate-volume roads in rural and suburban areas are usually unsignalized. These roadways are generally associated with high-speed travel and relatively lower geometrics than those in more developed suburban and urban areas.

General Description of 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 I-1).

Exhibit I-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 I-1 Fatal Crashes by Location

EXHIBIT I-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 an average of 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.

Objectives of the Emphasis Area

The objectives for improving safety at unsignalized intersections are explained below. Exhibit I-3 lists the objectives and strategies for improving safety at unsignalized intersections. Most of the objectives concern the physical improvement of unsignalized intersections and their approaches, while others relate to driver compliance. The strategies considered go across the full range of engineering, enforcement, and education. The physical improvements considered include both geometric design modifications and changes to traffic control devices:

• Improve management of access near unsignalized intersections—Driveway access at or near an unsignalized intersection may confuse drivers using the intersection and create vehicle-vehicle conflicts. For good access management, driveways within 250 feet of an intersection should be closed, relocated, or restricted.
• Reduce the frequency and severity of intersection conflicts through geometric design improvements—Reducing the frequency and severity of vehicle-vehicle conflicts at intersections can reduce the frequency and severity of intersection crashes. This can be accomplished by separating through and turning movements at the intersection, restricting or eliminating turning maneuvers, providing acceleration lanes, and closing or relocating intersections.
• Improve sight distance at unsignalized intersections—Some collisions at unsignalized intersections occur because of limited sight distance for drivers approaching the intersection or for drivers stopped at an intersection approach. Provision of clear sight triangles in each quadrant of an intersection can minimize the possibility of crashes related to sight obstructions.
• Improve availability of gaps in traffic and assist drivers in judging gap sizes at unsignalized intersections—Some collisions at unsignalized intersections occur because drivers have difficulty judging gap sizes before deciding whether to initiate a roadway entry or a turning maneuver. Drivers stopped to wait for the oncoming traffic stream often choose to proceed when oncoming vehicles are close, thus increasing the probability for a collision.
• Improve driver awareness of intersections as viewed from the intersection approach—Some intersection-related collisions occur because one or more drivers approaching an intersection are unaware of the intersection until it is too late to avoid a collision. This is a particular problem for drivers approaching unsignalized intersections on high-speed uncontrolled approaches. Improved signing and delineation and installation of lighting can help warn drivers of the presence of the intersection. In some situations where other measures have not been effective, rumble strips may be used to get the driver's attention.
• Choose appropriate intersection traffic control to minimize crash frequency and severity—The type of traffic control chosen for an intersection has a strong influence on the frequency and severity of crashes that occur at the intersection. The type of traffic control should be appropriate for the configuration of the intersection and the traffic volumes to be served. Unsignalized intersections generally have fewer crashes than comparable signalized intersections, so signalization should be avoided where practical. Alternatives to signalization that should be considered are two-way stop control (with or without flashing beacons), all-way stop control (with or without flashing beacons), and roundabouts.
• Improve driver compliance with traffic control devices and traffic laws at intersections—Many accidents are caused by noncompliance with traffic control devices or traffic laws at intersections. Enforcement has been shown to be an effective measure in reducing traffic-law violations and, consequently, in improving safety at intersections.
• Reduce operating speeds on specific intersection approaches—At certain high-speed intersection approaches, implementing speed-reduction measures may provide an approaching driver with additional time to make safer and more efficient intersection-related decisions. The speed-reduction measure will get the driver's attention and prepare the driver for making a stop or other appropriate action, thus potentially reducing right-angle and rear-end collisions.
• Guide motorists more effectively through complex intersections—As drivers approach and traverse through complex intersections, drivers may be required to perform unusual or unexpected maneuvers. Providing more effective guidance through the intersection, through the use of signing and pavement markers, will minimize the likelihood of a vehicle leaving its appropriate lane and encroaching upon an adjacent lane. The additional guidance will also minimize indecision by drivers, thus reducing the potential for conflicts.

EXHIBIT I-3
Objectives and Strategies for Improving Safety at Unsignalized Intersections