=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 pounds, THETA = 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 THETA_ _w_ = _W_ tan _THETA_ _mv^2_ _wv^2_ _F_ = ------- = ------ _R_ _gR_ If _F_ = _w_ there will be no tendency to skid; hence the rate of superelevation necessary in any case is as follows: _Wv^2_ _W_ tan _THETA_ = ------- _gR_ _v^2_ tan _THETA_ = ------- _gR_ The amount of superelevation required, therefore, varies as the square of the velocity and inversely as the radius of the curve.