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Rh Of the bridges over streams 1,281 are on urban sections of the system, 9,243 on rural sections. Of the bridges over railroads 396 are on urban sections, 583 on rural sections of the system. Of those over highways 217 are on urban sections; 167 on rural portions. Of the 161 remaining structures, 104 are on urban portions and 57 are on rural sections. Of the total of 12,048 structures carrying the interstate routes, 1,998 are on urban sections and 10,050 on rural sections of the system.

Additional to the 12,048 bridges which carry the interstate routes over streams, railroads, and other highways, there are 943 bridges, 562 on urban and 381 on rural sections of the system, which carry railroads over the interstate routes; and 336 structures, 185 on urban and 151 on rural sections, which carry other highways over the interstate routes.

The system, as generally described above, consists of those existing roads and streets now commonly used for travel along the general lines of the designated interstate routes. All of these roads and streets were reviewed in detail by the State highway departments. Field surveys were made, as necessary, and these were supplemented by reliable records to prepare an inventory of the physical conditions existing on all parts of the system and an enumeration of the traffic which each section served in 1948. These data were uniformly compiled by all of the State highway departments and reported in detail to the Public Roads Administration, where they were summarized for the system as a whole.

Figure 2 shows a sample of the graphical highway data sheet on which the desired information was reported by the State highway departments. Supplementary to the report in this form, the States coded many of the items in preparation for mechanical tabulation, and this coding was verified, and the data tabulated and analyzed, by the Public Roads Administration.

The following pages summarize the more significant information revealed by the survey.

TYPES OF SURFACES

All but 24 miles of the system as it exists at present have been surfaced. Of the small unsurfaced mileage, 9 miles are urban and 15 miles are rural. Of this unsurfaced mileage, all but one-half mile had been graded and drained at the time of the survey.

Low-type surfaces

A small mileage—6 miles, of which 2 miles are urban and 4 miles rural—is surfaced with selected soil. These are the most primitive surfaces now existing on the system. With 138 miles of untreated gravel or stone surfaces, 3 miles in urban and 135 miles in rural areas, they constitute a total of 144 miles classed as of low-type surface. Of this total, 5 miles are urban and 139 miles rural.

Intermediate-type surfaces

Surfaces classed as of intermediate type, consisting of three low-cost types of bituminous surfaces, exist on 4,990 miles of the system, of which 417 miles are in urban and 4,573 miles are in rural areas.