Left-turn and through movements often receive separate green time because their paths conflict. Conventional lane assignments may leave useful pavement underused while one movement waits and another is served. Tandem signals use an additional upstream control point to organize vehicles into successive groups before they reach the main intersection.
The film first shows an ordinary approach with a left-turn lane and two through lanes under conventional phasing. The second run uses a pre-signal and carefully managed lane changes so vehicles can be grouped for the next release. Through vehicles move across only when a safe gap is available, and the revised formation clears the heavier queue more quickly in the illustrated scenario. The lesson is that intersection capacity depends on how vehicles are sorted before the stop line as well as on green duration. Tandem operation gains space-time flexibility by moving part of the organization upstream, where conflicts can be handled deliberately.
Four green lights along a corridor do not guarantee a smooth trip. If their timing is unrelated to travel between intersections, a vehicle released by one signal may arrive at the next just as it turns red. The same road can therefore produce repeated stops without any change in demand or geometry.
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An intersection contains several traffic streams that would like to move at the same time. The problem of signal phase design is to allow compatible streams together while separating movements whose paths cross. The video turns that spatial safety rule into a graph problem.
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The center of a conventional intersection forces many turning paths to compete for the same space. A knotted intersection rearranges those paths and schedules passage through several smaller conflict areas, allowing movements to weave through a compact geometry in a controlled rhythm.
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