Consider the following sequence of diagrams. In each, like-colored triangles are similar. This follows because the alley walls are parallel.
By similar triangles, 9/h = h/4.
Hence h2 = 36.
Rejecting the negative root, h, the vertical height of the intersection above the ground, is 6 feet.
Source: Original, with thanks to S. Owen on wu :: forums for suggesting the extension from vertical to (parallel) sloping alley walls