The figure above shows the dimensions of a semicircular cross section of…

The figure above shows the dimensions of a semicircular cross section of a one-way tunnel. The single traffic lane is 12 feet wide and is equidistant from the sides of the tunnel. If vehicles must clear the top of the tunnel by at least ½ foot when they are inside the traffic lane, what should be the limit L on the height of vehicles that are allowed to use the tunnel?

This problem refers to a figure in its source book that isn't reproduced here.

AQuA-RAT — Algebra Question Answering with Rationales (Ling et al., DeepMind, 2017) · train #3874