Signalized Intersections

Introduction

One of the hallmarks of the AASHTO SHSP is to approach safety problems in a comprehensive manner. 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, the electronic form of the material uses hypertext linkages to enable seamless integration of various approaches to a given problem.

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 the formation of working groups and alliances that represent all of the elements of the safety system. In so doing, members of these groups can draw upon 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. Two guides in the NCHRP Report 500 series discuss intersections: this volume covers signalized intersections, and Volume 5 discusses unsignalized intersections. This implementation guide provides guidance to highway agencies that desire to implement safety improvements at signalized 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, and it is the crossing and turning maneuvers occurring at intersections that create opportunities for vehicle-vehicle, vehicle-pedestrian, and vehicle-bicycle conflicts. Thus, intersections are likely points for concentrations of traffic crashes. Although intersections constitute a very small portion of the highway system, crashes at intersections account for as much as half of all crashes in urban areas and about one quarter of crashes in rural areas.

Signalized intersections are generally the most heavily traveled intersection types and are therefore a major element of the highway fatality and crash problem nationally. Fatal crashes at signalized intersections are predominately multivehicle.

Signalized intersections are operationally complex, with many factors contributing to the potential safety problems. The intent of a signal is to control and separate conflicts between vehicles, pedestrians, and cyclists to enable safe and efficient operations. Operation of a signal itself, however, produces conflicts (e.g., conflicts between through vehicles that could lead to rear-end crashes). In addition, varying signal operations (timing and phasing) place demands on drivers that are not always met. While the focus of the strategies discussed in this guide is on reducing fatalities at signalized intersections, the implementation of many of these strategies will likely lead to an overall reduction in intersection crashes.