An existing highway-railway at-grade crossing is being redesigned as grade separated to improve traffic operations. The railway must remain at the same elevation. The highway is being reconstructed to travel under the railway. The underpass will be a sag curve that connects 2% tangent sections on both ends, and the PVI will be centered under the railway (a symmetrical alignment.) The sag curve design speed is 45 mph. How many feet below the railway should the PVI of the curve be located