Going 70mph:
1 minute = 1.1666 miles
1.1666 = 6159.648 feet
30 seconds = .583 miles
.583 miles = 3078 feet
Vertical clearance. Minimum vertical clearance under overhead structures (including over the paved shoulders) of 16 feet (4.88 m) in rural areas and 14 feet (4.27 m) in urban areas, with allowance for extra layers of pavement. Through urban areas at least one routing should have 16 feet (4.88 m) clearances. Sign supports and pedestrian overpasses must be at least 17 feet (5.18 m) above the road, except on urban routes with lesser clearance, where they should be at least 1 ft (0.3 m) higher than other objects.
Tunnel clearance. Tunnels should in theory be equivalent to long overcrossings, but because of cost the standards can be reduced. Vertical clearance is the same as under bridges, including the provision for alternate routing. Width should be at least 44 feet (13.41 m), which consists of two 12 feet (3.66 m) lanes, 10 feet (3.05 m) outside and 5 feet (1.52 m) inside shoulders, and 2.5 feet (0.76 m) safety walkways on each side. If necessary to meet the dimensions of the approach, this can be shifted left or right. A reduced width is acceptable due to high cost. In this case, the minimum width is 30 feet (9.14 m), with at least 2 feet (0.61 m) more than the approach for the sum of the shoulder widths, but at least 24 feet (7.32 m) total, and at least 1.5 feet (0.46 m) on each side for a safety walkway. If there is no safety walkway, a 3-foot (0.91 m) offset with a "safety shape" in the wall is acceptable.
