Page:Highways for the National Defense.pdf/30

29 indicated in the following schedule of widths corresponding to various classes of present average 24-hour traffic volume:

New and widened underpasses on three- and four-lane roads should have horizontal clearances at least 6 feet and preferably 20 feet wider than the width between outer edges of an approach pavement or pavements designed for 12-foot lanes.

Vertical clearance.—Existing bridges and underpasses having a vertical clearance of less than 12½ feet should be heightened as promptly as possible.

New and heightened bridges and underpasses should have vertical clearance of at least 14, feet. A vertical clearance of 14 feet is sufficient for the passage of all military equipment and substantially all civil vehicles. Existing bridges and underpasses affording vertical clearance of less than 14 feet, if not heightened, should have their minimum clearance plainly posted upon them.

Roads.

Surfaces and foundations.—All road surfaces should be dustless; and all surfaces and foundations should be designed in accordance with the present practice of each State for the all-weather support of repeated applications of 9,000-pound wheel loads on pneumatic tires, wheel load being defined as half the axle load where dual wheels are used.

Road surfaces designed in accordance with this recommended standard will be completely adequate for the accommodation of all normal civil traffic and all traffic of military vehicles, including all classes of tanks at present in use and projected.

Two-lane pavement width.—Existing two-lane roads having a pavement width of less than 18 feet, should be widened as promptly as possible.

Newly constructed and widened pavements on two-lane roads should be designed according to the following schedule of widths corresponding to various classes of present average 24-hour traffic volume: