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With the exception of the interstate system, the maintenance cost estimates appear reasonable. Lack of sufficient experience on the art of many States in maintaining the types of highway built to interstate system standards may have resulted in some underestimating of the costs on that system. In States, for example, where an appreciable mileage of divided highways now exists (100 miles or more), the estimated costs of maintaining mileage built to interstate system standards were higher than for other States which have had only limited experience in maintaining such facilities. Using the data from the more experienced States as a guide, it is probable that, on a nationwide basis, the estimates of maintenance needs for the interstate system as it will exist in 1965 and future years are understated by about 20 percent. Since it was somata: preferable to retain the estimates reported by the States to the maximum possible extent, no revisions have been made in the entries in table 5 to reflect this possible increase.

ADMINISTRATIVE COSTS

In addition to estimating future construction and maintenance costs for the several road systems, each State also prepared an estimate of those administrative costs which were not included elsewhere in the estimates. Highway patrol and other traffic police costs were not included.

Administrative costs were estimated to be slightly less than 5 percent of the combined total cost of construction and maintenance for all systems combined. Minor variations exist with respect to individual systems. The percentage was slightly lower than the average on local roads and streets and slightly above the average on the interstate system and other Federal-aid primary highways. Considerably wider variations existed between individual States. Such differences can be expected, however, because of varying practices among the States in accounting for administrative costs. Some elect