The purpose of this study was to determine the effectiveness of green-extension systems (GES) for reducing the dilemma-zone problem associated with the amber phase of traffic signals at high-speed intersections. Reactions of 2,100 drivers were noted during the amber phase at nine intersections, and the dilemma-zone distances with respect to the stop bar were determined.

Before-and-after studies made at three green-extension sites showed a 54-percent reduction in total accidents and a 75-percent reduction in rear-end accidents after GES installation. Accident severity was unaffected.

Conflict, volume, delay, and speed data were taken before and after GES installation at two sites. A 62-percent reduction in yellow-phase conflicts was noted after green extension was provided, and conflict rates decreased significantly at both sites. No significant change was found in vehicle delay due to green extension.

Expected present-worth benefits due to GES installations were found to range from $29,000 to $420,000, depending on the history of rear-end accidents. Benefit-cost ratios ranged from 6 to 70.

Report Date


Report Number

No. 503

Digital Object Identifier



For publication in the ITE Journal Institute of Transportation Engineers.