The remaining two tunnels in rural areas are the 531-foot Newcastle tunnel in California, and a 394-foot tunnel in Idaho. These have horizontal clearances of 30 and 20 feet, respectively. The vertical clearance of the Newcastle tunnel varies from 11½ to 20.7 feet; the minimum vertical clearance of the tunnel in Idaho is 15 feet. Hourly traffic in 1948 was 864 vehicles on the former and 171 on the latter.
TUNNELS IN URBAN AREAS
The Bankhead tunnel, a toll facility on US Route 90 in Mobile, Ala., is 3,389 feet long and has a minimum vertical clearance of 12 feet. Although carrying an hourly traffic in 1948 of 1,030 vehicles, and approached at one end by a 50-foot pavement, the tunnel is only 21 feet wide.
The 540-foot Yerba Buena tunnel in California lies between the two sections of the San Francisco-Oakland Bay toll bridge. This tunnel, part of US Route 40, carried 7,740 vehicles hourly in 1948. Average daily traffic for the year was 70,897 vehicles. The tunnel has two levels, the upper carrying six lanes of fast traffic in its 58-foot width. Vertical clearance on this level is 13 fect at the curb and 28.7 feet at the center. The lower level carries two interurban railway tracks and has a 31-foot, three-lane pavement for trucks. This section has a vertical clearance of 16 feet.
The Sumner tunnel in Massachusetts is a 5,635-foot toll facility under Boston Harbor. Approached by 63- and 83-foot pavements, it is only 21 feet wide and 12½ feet high. Its 1948 hourly traffic was 3,340 vehicles.
On New Jersey State Route 25 in Jersey City there is a depressed highway covered over for two-thirds of its 3,000-foot length. This semitunnel, with vertical clearance of 14} feet and horizontal clearance of 49 feet, carried 4,550 vehicles hourly in 1948.
At the west approach to the Lake Washington floating bridge in Seattle, there is a 1,466-foot twin-bore tunnel that carried 1,250 vehicles an hour in 1948. Each of the two bores is 23% feet wide and has a minimum vertical clearance of 14½ feet.
Other sizable urban tunnels are the 920-foot Beaucatcher tunnel in Asheville, N. C., and the 931-foot McCallie tunnel in Chattanooga, Tenn. These have horizontal clearances of 32 and 24 feet, and minimum vertical clearances of 14.4 and 20 feet, respectively; their hourly traffic in 1948 was 1,090 and 2,720 vehicles.
The smallest urban tunnel is in the town of St. George, Utah. Less than 500 feet long, it is 21 feet wide and 13% feet high, with an hourly traffic of 585 vehicles in 1948.
Traffic Volumes on the System
Average daily traffic varied in 1948 from less than 200 to more than 70,000 vehicles on various parts of the system. The variation of traffic density throughout the system is shown in figure 3.
MILEAGE CLASSIFIED BY TRAFFIC VOLUME
There were 359 miles on which the traffic averaged less than 400 vehicles daily, all but 1 mile on rural sections of the system and the single urban mile in a town of less than 5,000 population.
