I've always been fascinated by roads and traffic management. Optimizing traffic flow is quite a difficult problem, and well-suited to the fuzzy Artificial Intelligence domain. My wife sent me an article about some construction on the Highway 94/ Interstate 70 interchange that said that the new design will be a single-point urban interchange, also known as a SPUI.

spui-detail.gif diamond-detail.gif

In contrast to the more traditional diamond interchange, also shown above, the SPUI allows "opposing" left turns to happen simultaneously. The blue arrows in the diagrams show the left turns required to get onto the interstate, and the red arrows show the left turns required to get off of the interstate. As can be seen, the diamond interchange requires traffic following the two red arrows or the two blue arrows to cross each other, meaning that both arrows of the same color can't be given green lights at the same time. Thus, when the straight-through traffic is added in, the diamond interchange requires five light phases to give everyone a chance to go.

In contrast, the SPUI allows both red arrow turns to happen simultaneously, and likewise both blue arrow turns. With a phase for straight-through traffic, the SPUI requires only three light phases, thereby allowing up to 40% more capacity than a traditional diamond. Cool, huh?

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