Intersection Safety Case Study
Continuous Green T-Intersections
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Federal Highway Administration
This case study is one in a series documenting successful intersection safety treatments and the crash reductions that were experienced. Traffic engineers and other transportation professionals can use the information contained in the case study to answer the following questions:
Angle crashes are among the most severe crashes that occur in intersections, including T-intersections. In some cases, substandard sight distance can contribute to this problem. Several States including Colorado, Florida, Maryland, North Carolina, and South Carolina have converted from fully-signalized to continuous green T-intersections (CGT) to improve safety.
The following case study showcases two rural intersections in Colorado where the signal-controlled through lane on the flat side (top) of a T-intersection was converted to a CGT. The treatment was implemented to reduce angle crashes due to left-turning traffic on the stem, turning in front of the though movement on the top of the T (Figure 1 provides a photograph of one of the intersections).
Both of the intersections complied with minimum Manual on Uniform Traffic Control Devices (MUTCD) requirements before improvements. The CDOT converted both of these fully-signalized intersections to CGTs. The CGT design allows main line through traffic to pass through a signalized intersection without stopping (the top side of the "T"), while also eliminating conflicting vehicular movement. With a CGT, the through movement on the main line approach to the intersection is denoted by a steady green arrow traffic signal as well as by pavement markings or other lane delineation devices, so left-turning traffic stays in its respective lane (CDOT implemented advance warning signs to inform drivers of the special lane configuration). Engineers should only consider the CGT at intersections with three approaches, moderate-to-low left-turn volumes from the cross-street, and high arterial through volumes.
This case study examines two rural T-intersections in Colorado, with a high incidence of injury and angle crashes. Crash reduction results were based on a review of "before and after" data from these intersections during a period of four years, which occurred during the period between 1994 - 20061. (The "before" and "after" observation periods were 24 months at both intersections).
Problem: Two T-intersections in Grand Junction and Durango, CO were experiencing a high incidence of crashes, particularly angle crashes and many with injuries, due to limited stopping sight distance.
Solution: CDOT sought an intersection design treatment that would eliminate the stopping sight distance problem and to reduce the number of angle crashes, while also improving the efficiency of these intersections:
Table 1 summarizes the results of the "before" and "after" crash analysis at the treated intersections.
CDOT experienced no implementation issues converting these intersections to CGTs. However, the major movement of the side streets (the minor routes on the "stems" of the T-intersections) was affected by the lane closures required for maintenance of these CGTs, as there were no reasonable alternate routes. Also, the geometric changes (i.e., lane construction) required more time and approvals than operational changes (i.e., traffic signal timing adjustments).
The construction cost of the CGT, which included the new signal as well as the raised median work, was approximately $300,000 for each intersection.
The CGT was implemented within three months at each intersection.
The CGTs were effective in substantially reducing angle, injury and total crashes at these intersections.
Summary of Results
The safety enhancements discussed in this case study were added to reduce angle, injury and total crashes. Implementation of the CGT cumulatively reduced angle crashes at the treated intersections by 96.8 percent, injury crashes by 70 percent, and total crashes by 60 percent.
1) Jarem, E.S., "Safety and Operational Characteristics of Continuous Green Through Lanes at Signalized Intersections in Florida," Presented at the 2004 Institute of Transportation Engineers (ITE) Annual Meeting and Exhibit.
2) Boone, J.L., and Hummer, J.E., Unconventional Design and Operation Strategies for Oversaturated major Suburban arterials: Final Report, Project 23241-93-9, North Carolina State University, Department of Civil Engineering, Center for Transportation Engineering Studies, April 1995.
3) Reid, Jonathan, "Unconventional arterial intersection design, management and operations strategies," Parsons Brinckerhoff, July 2004.
Appendix A: US-50 and SH-141 Plan Details
1 Note that crash reductions in this report reflect the percent reduction per year based on the difference between the total number of "before" and "after" crashes.
2 Plan details for this intersection were not available.
For More Information
Visit FHWA's intersection safety web site to download this and other case studies highlighting proven intersection safety treatments from across the country.