High-type surfaces
By far the greater part of the system—32,642 miles—is paved with surfaces of high type. Of these surfaces 16,091 miles are high-type bituminous pavement, 2,390 miles in urban, and 13,701 miles in rural areas.
A mileage nearly as large—15,230 miles—is paved with portland cement concrete; 2,478 miles in urban, and 12,752 miles in rural areas. The remainder, 1,321 miles, is surfaced principally with brick or block pavements and different high-type surfaces in combination. Of this latter mileage, 670 miles are in urban, and 651 miles are in rural areas.
AGE OF SURFACES
While, as stated above, a very large part of the system is surfaced with pavements classed as of high type, and only an insignificant mileage remains unimproved, these facts of surface-type classification do not reflect the condition of the surfaces of the highways, and there is no convenient gage by which conditions so various as those of the surfaces existing on the system can be measured and intelligibly recorded and classified.
Average age
Perhaps the best indication of the condition of the surfaces may be found in their ages, which for all rural portions of the system are definitely known. The average age of all surfaces on rural sections of the system is 12 years. Many, of course, are much older. The roadways on which these surfaces are laid are nearly half again as old, averaging about 17 years of age. In other words, the average mile of rural highway on the interstate system consists of a surface that was last improved in 1937 and a roadway upon which the last major improvement of alinement and grade was made in 1932. These are most significant facts in view of the increases in traffic and the demands of safe operation that have occurred during these intervals.
Age by type of surface
Of the 27,104 miles of high-type pavement on rural sections of the system, 3,743 miles or 14 percent are more than 20 years old. Between 10 and 20 years of age there are 12,614 miles, or 46 percent of the total; and only 10,747 miles, or 40 percent, are less than 10 years old.
The surfaces of intermediate type average even older. Of the rural surfaced mileage of this class, totaling 4,573 miles, 614 miles, or 13 percent, are more than 20 years old; 2,698 miles, or 59 percent, are between 10 and 20 years of age; and only 1,261 miles, or 28 percent, are less than 10 years old.
The low-type surfaces on the system, being of less durable character, naturally are of lesser present age. Of the surfaces of this class, totaling 139 miles on rural sections of the system, only 3 miles, or 2 percent, are more than 20 years old; 50 miles, or 36 percent, are between 10 and 20 years old; and 86 miles, or 62 percent, are less than 10 years of age.
WIDTHS OF SURFACES AND SHOULDERS
Of the 5,969 route miles of the system in urban places, 695 miles are made up of pairs of one-way streets, the street mileage of which
