Yes, Race Resources that would be a very practical approximation method.
I started out thinking that there must be some simple way to do this exactly. After a bunch of smart people looked at it, turns out it IS an extremely difficult math problem. Because of that, it is nothing we are ever going to use. Today I was just trying to explain what the variable 'a' was. Dr. Coleman started using x1, y1 etc but it got so hairy so fast, he went to single character Cartesian points for simplicity, or a,b.
Yes, even knowing d (distance needed), r and y (half the width of the road) you still have to solve for 'a'. I am guessing that is what crashed his program.
I tried to get that equation to execute with 'a' as a known and solving for d. Somewhere in those 8 sets of nested parentheses, is something that JustBasic does not like.
It is a tough problem with a minimal payoff but my hat is off to anyone who can solve that equation for 'a'!