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66 The schedule referred to follows:

These quantities were understood to represent averages per mile and were not construed as applying to short sections of the most rugged topography. Quantities intermediate between the tabulated values were used where conditions warranted. The estimated unit cost of grading varied from $0.15 to $1 per cubic yard, the average being about $0.32 per cubic yard.

Retaining walls.—Where retaining walls were recommended, as in heavy mountainous areas, estimated quantities were determined between control points. Consideration was given to the use of viaducts in lieu of high fills and grading quantities were appropriately modified.

Drainage structures.—Costs of drainage structures up to 20-foot spans were determined at an estimated cost per mile between given control points.

Surfacing.—High-type surfaces of a thickness consistent with anticipated traffic requirements were estimated on the basis of 14,100 square yards per mile for two-lane sections and 28,200 square yards per mile for four-lane sections. ‘The unit costs varied from $0.65 to $3.30 per square yard, depending upon the type of surfacing used.

Miscellaneous.—Areas of clearing and grubbing were estimated to vary from about 8 acres per mile for two-lane sections to about 25 acres per mile for four-lane sections.

Curbs, where required to control drainage, were placed 2 feet outside the normal road surfacing and the estimated cost per mile included the cost of the extra 2 feet of pavement.

Guard railing was estimated where slopes as steep as 1 on 2 were required.

Large reflecting signs were provided in repetition approaching all accesses. A maximum amount of $1,000 per mile of highway regardless of width was allowed for this item. Fencing, on both sides of the highway, was estimated at an average of $3,000 per mile.

Topsoiling, seeding, and planting costs depend to a considerable degree on topography and climate. Allowances were made for this item in the amounts of $5,000 to $8,000 per mile in suburban areas, and $3,000 to $6,000 per mile in rural areas.

Bridges, viaducts, tunnels, and accesses.—The width of bridges was defined as the perpendicular distance face to face of the bridge; the length as the distance face to face of the abutments measured parallel to the centerline of the road. An undercrossing was defined as one at which the selected route would pass under, and an overcrossing as