Madrid Math

Continuing a run of four 4, 2 mathematically significant roadsigns is this great submission from Spain by RMC. Road Sign Math appears to be a universal language. While traveling on M-40, which rings Madrid, he found this winner on a gantry over the road. He whipped out his camera phone and snapped this picture while driving.



We seem to be stuck for a moment on this basic equation (Kettle Creek, North Dakota Root, Odd Little Sign) and now Spain.

$$\sqrt{5 - 1} = 2$$

This sign is found on M-40, in the east sector, heading north.