This was one of my favourite things to do in AC, however recently updates to CSP seem to have broken it. You can get past the "Initalising AI Spline" screen by waiting around 20 minutes, after that it should be quicker to load (a few seconds for me with a good 20ish AI cars).
The AI don't seem to drive as quickly as they used to and are stuck following each other in a straight line sometimes, or will not go to certain areas they used to race in.