The Problem of Design
It will be convenient to consider separately the components of a road design, although in the actual design the consideration of these cannot be separated because all parts of the plan must fit together.
Alignment.—The alignment of the road is determined to a considerable extent by the existing right-of-way, which may follow section lines, regardless of topography, as is the case with many roads in the prairie states, or it may follow the valleys, ridges, or other favorable location in hilly country. In many places the roads of necessity wind around among the hills in order to avoid excessive grades. In designing an improvement, it is generally desirable to follow the existing right-of-way so far as possible. But the element of safety must not be lost sight of, and curves should not preclude a view ahead for sufficient distance to insure safety to vehicles. The necessary length of clear view ahead is usually assumed to be 250 feet, but probably 200 feet is a satisfactory compromise distance when a greater distance cannot be obtained at reasonable cost. To secure suitable sight distance, the curves must be of long radii, and where possible the right-of-way on the inside of the curve should be cleared of trees or brush that will obstruct the view. Where the topography will not permit a long radius curve and the view is obstructed by an embankment or by growing crops or other growth, it is desirable to separate the tracks around the curve to eliminate the possibility of accidents on the curve. This is readily accomplished if the road is surfaced, but if it is not surfaced, the same end is accomplished by making the earth road of ample width at the curve.
Relocations should be resorted to whenever they shorten distances or reduce grades sufficiently to compensate for the cost.
Intersections.—At road intersections, it is always difficult to design a curve that entirely meets the requirements of safety because there is not enough room in the right-of-way, and enough additional right-of-way must be secured to permit the proper design. It is not necessary to provide an intersection that is adapted to high speed traffic, where main roads cross, but, on the contrary, a design that automatically causes traffic to slow up has distinct advantages.
Where a main route, improved with a hard surface, crosses secondary roads, it is satisfactory to continue the paved surface across the intersecting road at normal width and make no provision for the intersecting road traffic other than a properly graded approach at the intersection.
Superelevation.—On all curved sections of road, other than intersections, account is taken of the tendency of motor cars to skid toward the outside of the curve. This tendency is counteracted by designing the cross section with superelevation.
Fig. 6
In Fig. 6, F represents the tangential force that tends to cause skidding. W represents the weight of the vehicle in pounds, θ = the angle of superelevated surface c-d, with the horizontal c-a. R represents the radius of the curve upon which the vehicle is moving. w is the component of the weight parallel to the surface c-d, v = velocity of the vehicle in feet per second. m = mass of vehicle = W⁄g θ