cases at least 3½ feet and preferably 4½ feet outside the edge of the pavement.
Sidewalks shall be provided on bridges wherever the volume of pedestrian traffic justifies.
At underpasses, the lateral clearance between the right edge of the pavement of through-traffic lanes and abutments, walls, or piers shall be at least 6 feet. The lateral clearance between the left edge of the pavement and center piers shall be at least 4½ feet.
Deficiencies of Existing Roads and Bridges
When the various conditions existing on the roads, streets, and bridges which presently constitute the designated interstate system are compared with the foregoing defined standards, it is found that a very large mileage of the roads and streets and most of the existing bridges are in some respects inadequate.
MUCH IMPROVEMENT NEEDED
Of the 31,831 miles of roads in rural areas some improvement is required on all but 1,900 miles. Of the 5,969 miles of roads and streets in urban areas all but 398 miles require some improvement. Of the 10,050 bridges carrying rural sections of the system only 483 are completely adequate by the defined standards. The degree of the deficiency of roads and structures in some measure inadequate varies widely, from slight to serious.
SURFACE TYPE DEFICIENCIES
The choice of surface types on the system is reasonably consistent with the volume of traffic carried by the various sections.
Low-type surfaces
The 24 miles unsurfaced and 144 miles surfaced with low-type surfaces are, of course, clearly deficient. A substantial part of these mileages carries traffic in excess of 2,000 vehicles a day. But even on the sections of somewhat lighter traffic, considering that they are part of the country’s major network, low-type surfaces are out of place.
Intermediate-type surfaces
The existing surfaces of intermediate type, found on 4,990 miles of the system, are mostly on sections of relatively low traffic volume.
Only about 91 miles, or about 2 percent, of the total of 4,573 miles of rural roads surfaced with pavement of intermediate type carry as many as 5,000 vehicles per day. Only 568 rural miles, about 12 percent of the total, carry 3,000 or more vehicles a day. The choice of surface for the 91 miles may be seriously questioned; the additional 477 miles carrying traffic between 3,000 and 5,000 vehicles per day may be said to border upon inadequacy.
The situation in the urban areas is less favorable. Of the total of 417 miles of urban sections paved with intermediate types of surface, 26 percent carry traffic of 5,000 vehicles or more daily; 48 percent carry 3,000 or more vehicles a day.
Of the 93 miles with traffic between 3,000 and 5,000 vehicles daily, 27 miles are in towns of more than 5,000 population. At least this
