BRIDGE CONSTRUCTION REQUIRED
On the 11,891 miles of highway relocation proposed, there are 4,893 bridges that would have to be constructed. Of these, 2,693 are on the relocated rural sections and 2,200 are on the urban relocations.
On the sections of the system to be improved on approximately the present locations, totaling 22,970 miles, and on the 2,298 miles otherwise adequate, there are 5,925 bridges which require improvement in some degree, some by outright reconstruction. Of these 5,187 are on rural sections and 738 on urban sections of the system.
TUNNEL CONSTRUCTION
Of the 13 existing rural tunnels, 6 of the least adequate will be avoided in the proposed improvement of the system by relocation of the existing approach roads. The remaining seven, all adequate in vertical clearance, will be supplemented in the proposed improvement by parallel bores. The twin bores, then of adequate width, will each carry one direction of traffic.
Improvement of urban tunnels
Of the eight existing urban tunnels, all but one are proposed for enlargement by the State highway departments. These are the Yerba Buena tunnel in California; the Sumner tunnel in Boston, Mass.; the semitunnel in Jersey City, N. J.; the Beaucatcher tunnel at Asheville, N. C.; the McCallie tunnel in Chattanooga, Tenn.; the tunnel in St. George, Utah; and the Lake Washington tunnel in Seattle, Wash.
The one existing urban tunnel for which no improvement was proposed by the State highway department is the Bankhead tunnel at Mobile, Ala. This tunnel, with its slightly restricted vertical clearance and 21-foot clear width, for an hourly traffic of over 1,000 vehicles in 1948, is regarded by the Public Roads Administration as definitely inadequate. The existing tunnel was built in 1940 at a cost of about $4,000,000. Its inadequacy of width, at least, should be corrected by the construction of a parallel tube, which should provide a minimum vertical clearance of 14 feet. For this purpose an estimate of $5,500,000 has been added to the State highway department’s estimate oft costs.
New tunnels proposed
In addition to the required work of enlargement of existing tunnels, construction of five new tunnels is proposed, of which one is on a rural section of the system and four are in urban areas.
The one new rural tunnel proposed is on U S Route 40 in California at the approach to the Carquinez Strait Bridge.
The four proposed new tunnels in urban areas are a 900-foot tunnel in Louisville, Ky.; a 700-foot expressway tunnel in Boston, Mass.; a 2,400-foot freeway tunnel in Kansas City, Mo.; and a new Palisades tunnel in New Jersey.
TRAFFIC SERVICE OF THE IMPROVED SYSTEM
The 2,298 miles of the system on which no road improvement is required, served in 1948, on the 1,998 rural miles included, an average
