mileage and the 109 miles carrying upward of 5,000 vehicles daily are of doubtful adequacy.
High-type surfaces
The bulk of the system, 5,538 miles in urban areas and 27,104 miles in rural areas, is paved with high-type surfaces. On the whole, the choice of pavement type as represented in the existing improvements on the system is appropriate.
Other factors
But a mere classification of surface types does not clearly establish the structural sufficiency of the existing surfaces. Many miles of the surfacing of appropriate type are doubtless of insufficient depth, and the foundation support of a large mileage is probably untrustworthy. The survey that has been made could not, in the limited time available, definitely establish the facts in this regard.
Age of surfaces
The age of the surfaces, previously mentioned, is the best available indication of the probability of a considerable measure of surface inadequacy. The average age of the rural surfaces of all types is 12 years; the average age of roadways on which they are laid is 17 years. Much of the mileage is, of course, of greater age than these averages.
The age of the rural mileage by 10-year groups is shown graphically in figure 4. As this graph also indicates, about 57 percent of the existing surfaces will probably wear out and require replacement in the
