next 10 years. This projection is based upon a knowledge of the life span of surfaces of all types, gained from a wide study of past surface replacement, and takes into account all of the conditions that contribute to retirement, of which physical deterioration is only one. The same basis underlies the prediction that about 52 percent of the high-type pavements on the system will require replacement by 1959, and that 85 percent of the intermediate-type surfaces and 93 percent of the low-type surfaces will be retired in the same 10-year period.
RATE OF FUTURE IMPROVEMENT
If surfaces of the system are retired in the future at a rate no greater than that of the past, all but the newest and most durable of the now existing surfaces will be replaced in the next 20 years. In view of the more pressing demand for correction of obsolescence already felt and likely to increase in urgency in future years, this is a conservative prediction; but it may serve to set a maximum limit upon the time in which all of the presently recognized deficiencies of the interstate system should be corrected. As the present surfaces are replaced, at least, if at no faster rate, the widths, alinements, sight distances, grades, and all other features of the routes of the system should be brought to the standards recommended in this report as desirable. The building of new surfaces on these routes at any time hereafter without at the same time correcting the serious geometric defects that exist, will be an indefensible prolongation of obsolescence.
DEFICIENCY OF SURFACE AND SHOULDER WIDTH
The present inadequacy of surface width is a much more serious deficiency of the system than the inadequacy of surface type. More than two-thirds of the rural mileage has surfaces that.are too narrow.
Substandard widths on rural roads
The minimum surface width complying with the defined standards under any conditions is 22 feet, and this applies only where the hourly traffic is less than 300 vehicles. The existing surface width is less than 22 feet on 17,746 miles of the system in rural areas, which is more than 55 percent of the total rural mileage. These are not all lightly traveled roads. The hourly traffic volume is less than 300 on only about 9,868 miles of these narrow roads, and is 300 or more vehicles on the remaining 7,878 miles. In fact, about 600 miles of these rural roads with surfaces less than 22 feet wide have hourly traffic volumes of 800 or more vehicles and should be improved as four-lane divided highways.
Surface width of 24 feet is required by the standards for traffic volumes of 300 to 800 vehicles an hour; and multiple-lane divided roads are required for volumes of 800 vehicles or more—four 12-foot lanes for traffic from 800 to 3,000 vehicles, and six lanes for traffic of 3,000 vehicles or more. Three-lane roads and undivided highways of four lanes or more are not desirable and have no place in the standards; however, for hourly traffic volumes of less than 800 vehicles, where two lanes would suffice, they are considered adequate.
Multiple-lane roads needed
On the basis of these standards and the already experienced traffic of 1948, there is a total of about 9,250 miles of the two-lane rural
