Improvement of secondary or farm-to-market roads and elim ination of highway-railroad grade crossings are being continued as parts of the Federal-aid program. The Federal Government generally pays half the cost of the highway work and the full cost of grade-crossing elimination.
State highway systems are constructed with State funds only and with State and county or other local funds.
Of the total State highway mileage, in 1936, there were 289, 103 miles surfaced. Of the surfaced mileage, 17,364 miles are of sand-clay and top soil; 103,958 miles are of gravel, chert, and shale (treated and untreated) ; 33,805 miles are low-cost bitu minous mix; 20,281 miles are waterbound macadam (treated and untreated) ; 14,232 miles are of bituminous macadam by penetration; 15,10o miles are of sheet asphalt, and bituminous concrete ; 81,283 miles are Portland cement concrete; 2,812 miles are of vitrified brick; 268 miles are of asphalt block, wood block, stone block, and miscellaneous types.
Of the unsurfaced mileage, amounting to 51,057 miles, 32,520 miles are unimproved or only partly graded, and 18,537 miles are improved to established grade and drained.
By far the larger part of the unimproved mileage of the country is in the county and local road system. There are hundreds of thousands of miles on which the traffic is so small that improve ment is not justified. Of the local road mileage approximately
860,90o miles have been surfaced; in large part these surfaces are of such low types as sand-clay, topsoil, gravel, and water bound macadam. In recent years great improvements have been made in methods of combining asphalt and tar with materials of a sandy or gravelly nature to make low-cost dustless surfaces. Such surfaces are particularly suited to secondary roads and a considerable mileage has been constructed.
Administration of Federal-aid Road Construction.—Fed eral-aid roads are constructed under the immediate supervision of the several State highway departments subject to the approval of the Federal Works Administrator, who has delegated the details of administration to the Public Roads Administration. The ad ministration has established 12 districts, each in charge of a dis trict engineer who co-operates directly with the State highway engineers of the States included in his district. The four western districts are under the general supervision of a chief, with head quarters at San Francisco, who is responsible to the Commissioner at Washington. All other districts are directly under the super vision of the Commissioner.
When a State highway department desires Federal aid in financ ing a road on the Federal-aid system, it notifies the district engi neer, who, if he has not already done so, immediately makes an examination of the road and the plans proposed for its improve ment. If he approves the State's proposals he submits the project either to the chief at San Francisco or to the headquarters at Washington with appropriate recommendations. However, he may at once authorize the State highway department to proceed with the construction subject to subsequent agreement or any modifications of plans which may be required by his superior authorities.
A formal agreement is made between the State and the Federal Government. Construction then proceeds with full authority of the Government under the immediate supervision of State high way engineers and subject to frequent inspection by Federal en gineers.