Signalized Intersections

Executive Summary

Introduction

One of the hallmarks of the AASHTO Strategic Highway Safety Plan (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. As more guides are developed for other emphasis areas, the extent and usefulness of this form of implementation will become ever 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 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. 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.

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. Intersections constitute only a small part of the overall highway system, yet intersectionrelated crashes constitute more than 50 percent of all crashes in urban areas and over 30 percent in rural areas (Kuciemba and Cirillo, 1992). Just under a quarter of fatal crashes occur at intersections.

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.

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 I-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 I-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 I-1 Fatal Crashes by Relationship to Junction
Source: 2002 FARS data. Other relationships to junctions include 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 othervehicles.

EXHIBIT I-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.

Objectives of the Emphasis Area

The objectives for improving safety at signalized intersections are explained below. Exhibit I-3 lists the objectives and the related strategies for improving safety at signalized intersections. The strategies considered go across the full range of engineering, enforcement, and education. Physical improvements include both geometric design modifications and changes to traffic control devices:

• Reduce frequency and severity of intersection conflicts through traffic control and operational improvements—Improvements to the method of assigning right-of-way at signalized intersections can reduce the potential for conflicts. This can be accomplished by modifying signal phasing, providing additional traffic control devices and pavement markings, and restricting turn movements. Improvements to traffic control can also benefit traffic operations and reduce emergency response time.

• Reduce frequency and severity of intersection conflicts through geometric 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 incorporating geometric design solutions that separate through and turning movements at the intersection, restrict or eliminate turning maneuvers, and close or relocate intersections.

• Improve sight distance at signalized intersections—Provision of clear sight triangles in each quadrant of an intersection can minimize the possibility of crashes related to sight obstructions.

• Improve driver awareness of intersections and signal control—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. 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.

• Improve driver compliance with traffic control devices—Many accidents are caused or aggravated by drivers' noncompliance with traffic control devices or traffic laws at intersections. Both public education and enforcement have been shown to be effective in reducing traffic-law violations and consequently improving safety at intersections. Automated enforcement of traffic signals and speed limits is an increasingly common and cost-effective approach to improving driver compliance with traffic laws. At certain highspeed intersection approaches, implementing speed-reduction measures may provide an approaching driver with additional time to make safer intersection-related decisions.

• Improve access management near signalized intersections—Navigation, braking, and decision-making on intersection approaches creates additional workload on the driver. The presence of driveway access at or near a signalized intersection may confuse drivers using the intersection and create additional vehicle-vehicle conflicts. Measures to restrict driveways and to preclude cross-median turning movements in close proximity to signalized intersections can effectively reduce or eliminate serious multivehicle conflicts.

• Improve safety through other infrastructure treatments—Other improvements can be made to the intersection to decrease frequency and severity of crashes at signalized intersections. These include improving pavement conditions, coordinating operation of signals near railroad crossings, and moving signal hardware out of the clear zone.

EXHIBIT I-3 Emphasis Area Objectives and Strategies

Objectives	Strategies
17.2 A Reduce frequency and severity of intersection conflicts through traffic control and operational improvements	17.2 A1 Employ multiphase signal operation (P, T) 17.2 A2 Optimize clearance intervals (P) 17.2 A3 Restrict or eliminate turning maneuvers (including right turns on red) (T) 17.2 A4 Employ signal coordination (P) 17.2 A5 Employ emergency vehicle preemption (P) 17.2 A6 Improve operation of pedestrian and bicycle facilities at signalized intersections (P, T) 17.2 A7 Remove unwarranted signal (P)
17.2 B Reduce frequency and severity of intersection conflicts through geometric improvements	17.2 B1 Provide/improve left-turn channelization (P) 17.2 B2 Provide/improve right-turn channelization (P) 17.2 B3 Improve geometry of pedestrian and bicycle facilities (P, T) 17.2 B4 Revise geometry of complex intersections (P, T) 17.2 B5 Construct special solutions (T)
17.2 C Improve sight distance at signalized intersections	17.2 C1 Clear sight triangles (T) 17.2 C2 Redesign intersection approaches (P)
17.2 D Improve driver awareness of intersections and signal control	17.2 D1 Improve visibility of intersections on approach(es) (T) 17.2 D2 Improve visibility of signals and signs at intersections (T)
17.2 E Improve driver compliance with traffic control devices	17.2 E1 Provide public information and education (T) 17.2 E2 Provide targeted conventional enforcement of traffic laws (T) 17.2 E3 Implement automated enforcement of red-light running (cameras) (P) 17.2 E4 Implement automated enforcement of approach speeds (cameras) (T) 17.2 E5 Control speed on approaches (E)
17.2 F Improve access management near signalized intersections	17.2 F1 Restrict access to properties using driveway closures or turn restrictions (T) 17.2 F2 Restrict cross-median access near intersections (T)
17.2 G Improve driver compliance with traffic control devices	17.2 G1 Provide public information and education (T) 17.2 G2 Provide targeted conventional enforcement of traffic laws (T) 17.2 G3 Implement automated enforcement of red-light running (cameras) (P) 17.2 G4 Implement automated enforcement of approach speeds (cameras) (T) 17.2 G5 Control speed on approaches (E)
P = proven; T = tried; E= experimental. A fuller explanation of P, T, and E appears in Section V. Several strategies have substrategies with differing ratings.