Find the length between the two road segments. Refer to road data or assign someone to measure the distance between the road segments if needed. Call this distance "L."
Calculate the slope of each segment in terms of grade. Grade refers to the fractional increase in slope per unit. For example, you may find that the first segment is rising 1 inch per foot, which is 1/12 increase. Call the slopes for the first and second segments "s1" and "s2," respectively.
Find the change between the two slopes. Label this change "C." Use the formula C = s1 -- s2.
Write the design of the curve in mathematical terms. The equation will be a standard parabola in the form y = ax^2 + bx + c. The equation for the vertical curve is y = s1*x + C*L*x^2/200. Plug in the calculated variables to get the precise curve.