Willis Lamm's
Traffic Signal Collection

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"Dog House" Signal Sequences

Part Two

Leader left turn sequence -
Opposing left turns moving.

View the leader overlap sequence on YouTube.

Modern variations. With more modern traffic actuated controllers doghouse sequences can change according to traffic demands. For example, a signal may be programmed for opposing leader protected left turns. If twice as many vehicles are backed up in one left turn pocket as compared with the left turn pocket on the opposing side of the intersection, a controller could be programmed to terminate the leading protected left turn in the direction of the fewest vehicles once they clear the intersection, but hold over the protected left turn for in the direction where vehicles are still backed up in the left turn pocket. In such situations the direction being terminated will be given red indications, while the direction still permitted to flow will be overlapped with a through green ball - allowing the protected left turns to continue while corresponding (not oncoming) through traffic will be also allowed to go. Some controllers also allow "permissive intervals." These are intervals occur when the controller senses vehicles stacking up in a left turn pocket and a gap in opposing (oncoming) traffic. The controller might give the oncoming lanes a brief red interval while the vehicles in the left turn pocket are given an "extra" protected left turn interval. Correct programming of permissive intervals is critical to prevent what is known as the "Yellow Trap." The Yellow Trap occurs when vehicles are out in the intersection waiting for openings to make unprotected left turns when they catch a yellow light. Motorists habitually assume that when the light changes on them, oncoming traffic is also being given a yellow, then red light, so the left turning drivers try to clear the intersection. However, if it turns out that opposing traffic is not being given a red light but instead is being given a permissive interval, this oncoming through traffic will not be stopped and vehicles trying to complete left turns when the light changed on them could be broadsided. A feature will be developed shortly that more completely discusses the Yellow Trap and what can be done to avoid it.