Safety Evaluation of Continuous Green T Intersections

CHAPTER 1. INTRODUCTION

BACKGROUND ON CGT INTERSECTION

The Federal Highway Administration’s (FHWA) Development of Crash Modification Factors (DCMF) program was established in 2012 to address highway safety research needs for evaluating new and innovative safety strategies (improvements) by developing reliable quantitative estimates of their effectiveness in reducing crashes. The goal of the DCMF program is to provide measures of their safety effectiveness and benefit-cost (B/C) ratios for new safety strategies based on research. Promotion of the effective safety strategies has the potential benefit of decreasing total crashes and, subsequently, reducing fatalities. Furthermore, transportation agencies will be able to use safety effectiveness estimates and B/C ratios to manage safety on the highway and street network by making effective use of limited resources. There are 40 State transportation departments that provide technical feedback on safety improvements to the DCMF program and implement new safety improvements to facilitate evaluations. These States are members of the Evaluation of Low Cost Safety Improvements Pooled Fund Study (ELCSI-PFS) and have selected this study to be conducted under this program.

At-grade intersections are an inherent conflict location on the highway and street network because the turning or crossing paths of motorized and non-motorized users frequently interact at these locations. As a result, crashes involving both user groups often occur at intersections. FHWA estimates that, on average, 26 percent of fatal and 50 percent of injury crashes in the United States occur at intersections. National Cooperative Highway Research Program (NCHRP) Report 500, Volume 12, A Guide for Reducing Collisions at Signalized Intersections estimated that approximately 30 percent of fatal intersection crashes occur at locations with signalized control.⁽¹⁾ Intersection safety is a priority among transportation agencies in the United States.

Alternative intersection designs have emerged in recent years to improve traffic operations and safety. Implementation of specific alternative intersection forms is dependent on the conditions present at the location of interest. The presence of traffic congestion, high crash frequencies, or severe crash outcomes at existing intersections often necessitates either operational or safety improvements. Rather than seeking traditional traffic measures to mitigate delay or traffic safety problems, practitioners are now seeking opportunities to convert conventional intersections into alternative, innovative forms. Examples of alternative intersections include the displaced left-turn, restricted crossing U-turn, and median U-turn.

Another alternative intersection type that has been employed in several States is the continuous green T (CGT) intersection. CGT intersections are an alternative to conventional signalized T intersections. CGT intersections are characterized by a channelized left-turn movement from the minor street approach onto the mainline (major street), along with a continuous mainline through movement that occurs at the same time.⁽²⁾ The continuous-moving through lanes are not controlled by a traffic signal phase, while the other intersection movements are controlled by a three-phase signal. The through lanes on the mainline that have continuous flow typically contain a green through arrow signal indicator to inform drivers that they do not have to stop. The continuous through lanes are often separated from the left-turn and merge lanes with delineators, curbed islands, pavement markings, or other separations. Figure 1 shows a major street approach to the continuous through lanes of a CGT intersection. An aerial view of a full CGT intersection is shown in figure 2.

Figure 1. Photo. Driver view of approach toward continuous through lanes at CGT intersection (latitude: 32.210420, longitude: -80.695000).

Figure 2. Photo. Overhead view of approach toward continuous through lanes at CGT intersection (latitude: 32.240866, longitude: -80.816626).

BACKGROUND ON STUDY

In 1997, the American Association of State Highway and Transportation Officials (AASHTO) Standing Committee on Highway Traffic Safety, with the assistance of FHWA, the National Highway Traffic Safety Administration, and the Transportation Research Board Committee on Transportation Safety Management, met with safety experts in the fields of driver, vehicle, and highway issues from various organizations to develop a strategic plan for highway safety. These participants developed 22 key emphasis areas that affect highway safety. NCHRP published a series of guides to advance the implementation of countermeasures targeted to reduce crashes and injuries. Each guide addresses one of the emphasis areas and includes an introduction to the problem, a list of objectives for improving safety, and strategies for each objective. Each strategy is designated as proven, tried, or experimental. Many of the strategies discussed in these guides have not been rigorously evaluated; about 80 percent of the strategies are considered tried or experimental.

In 2005, to support the implementation of the guides, FHWA organized a PFS to evaluate low-cost safety strategies as part of this strategic highway safety effort. Over the years, the ELCSI-PFS has grown in size and now includes 40 States. The purpose of the ELCSI-PFS is to evaluate the safety effectiveness of tried and experimental, low-cost safety strategies through scientifically rigorous crash-based studies. The use of CGT at signalized intersections was selected as a strategy to be evaluated as part of this effort.