Unit Curve for Design of Highway Transitions

Calculations required for the design of a transition (spiral) curve are often facilitated by the use of tables that give the elements of the curve for selected values of length and minimum radius of the transition. The use of such tables can be dispensed with and the design done more simply by the use of a master curve called the unit curve. The elements of the unit curve are developed and listed in this paper. Every element (linear dimension) of a field highway transition, including the tangent distance, is obtained from the corresponding element of the unit curve as a multiple of the characteristic constant of the field curve. This characteristic constant is 2RL, in which R = minimum radius, and L = length of the transition. The design of a transition thus becomes a matter of picking up the listed values of the unit curve for the required elements and multiplying by the characteristic constant of the field curve. An example is included to explain the procedure.