One last point about CNU's proposal to encourage street connectivity. CNU proposes using a metric of 150 intersections per square mile. This is the wrong metric. If we want to mandate connectivity, we should use a metric that guarantees connectivity; requiring a certain number of intersections per square mile doesn't do the trick.

The street plan below illustrates how a developer could meet CNU's intersection-density standard without providing even minimal connectivity. The box is one mile by one mile. The black lines are streets and the white spaces in between are for houses and green belts.

It's crude, I know, but it makes the point. A developer could simply cut the interior of the subdivision into disjoint pods and cover each pod with a dense grid. I've drawn in the local streets for the pod in the lower left-hand corner. The blocks are supposed to be roughly 400-feet long, with one half-block. It has 26 intersections, which would scale up to 156 intersections for the entire subdivision. Even this density doesn't guarantee connectivity within the pod. This pod is divided into four quarters, which are mutually accessible only along the pod's perimeter road.

In this example, there are neither east-west nor north-south routes through the subdivision interior. The only through routes lie one mile apart along the subdivision's perimeter. I don't think is what CNU has in mind when it talks about connectivity.

If we are after connectivity we should use a metric that guarantees connectivity. We could do this by requiring a minimal number of independent routes -- that is, routes without common streets -- between any two nonadjacent intersections.* (For intersections interior to a street grid, that number is four.) By also requiring a minimal number of subdivision entrances per mile (say every 400 or 600 feet), we would get the desired through connectivity and internal connectivity without having to worry about creative subdivision street layouts. On the contrary, developers would get a bit more flexibility in laying out their subdivisions while providing the public benefit that is the point of this exercise.

*It isn't hard to determine the number of independent routes between two non-adjacent intersections. It's equal to the minimal number of separating intersections -- i.e., the minimal number of intersections you'd have to eliminate to disconnect the original intersections. This means it's enough to look for choke points. For example, the intersections interior to the pod sketched above would be disconnected from the rest of the subdivision by eliminating the intersections at the pod entrance. Hence they wouldn't satisfy a standard requiring more than two independent paths.

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