n 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.

[Illustration: Fig. 6]

In Fig. 6, _F_ represents the tangential force that tends to cause
skidding. _W_ represents the weight of the vehicle in poun