The smaller green circle is a complete circle of the radius of curvature of the curb. Larger orange circle would be the other 3/4 of 1' wider path ridden. "25 HIGH" (height of a right triangle) is distance from Continuous Street curb to a line running through the actual turnaround point.
Very astute, also, Admin: I do not know what Dr. Coleman is calling "a" either. I need to ask.
After seeing that it was not a simple problem, I have been coding to calculate the distances for known angles. Am in the process of trying to condense that code which is based on a different approach. Major value of the Coleman Equation is that it shows if a college prof does not say, "Hey, that's simple" then it is going to take a while. Fun exercise even if mainly academic.