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The 7,875 concrete bridges on the system are the most numerous of any single material, and their aggregate length, 191 miles, is slightly greater than that of the steel bridges. Only 149 of the concrete bridges are between 100 and 200 feet in length and only 93 are 200 feet and longer. Only 232 of the concrete bridges are of load capacity less than H15.

Twelve hundred and forty-five of the existing bridges are wooden structures, their aggregate length 29 miles. There are 47 of these bridges that are rated as of H20 capacity or more. Many are much weaker; in fact, the number of wooden bridges rated below H15 capacity, a total of 256, exceeds the number built of any other material.

Ten-foot vertical clearance and width narrower than the approach pavement, with the added hazard of a center pier, bottleneck the 13,000 vehicles a day, including 2,700 trucks and busses, that use this underpass on U S Routes 12, 14, and 151 in Wisconsin.

Of material other than steel, concrete, or wood there are 127 existing bridges, mostly of stone masonry. These have an agegregate length of. only 7 miles. Nine are between 100 and 200 feet and 14 are 200 feet or more in length. Only two are rated at less than H15 capacity.

The aggregate length of all of the 12,048 bridges on the system is 2,194,592 feet, or about 416 miles.

Tunnels on the System

There are presently 21 tunnels on the interstate system as designated. Thirteen of these are on rural sections of the system; eight are on urban sections.

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