On stephenson simutrans game we have several occurences, where private cars end up looping a few tiles there and back, or can be even tricked into dead end. The trick is find a busy route junction, and delete one so that there still is an alternative (but a detour). The remaining route will come to standstill, and cars will start to pile up where the original route diverged. The wont continue, even if the alternative road is clear. You can try at stephensonsiemens - delete the road at (575,50) - (please do it offline). If you rebuild the old route, it will be used immediately again.
I tried that on less busy roads, and the cars seemed to adapt to new route much better.
Solution? - if the directions found by private car tell him to turn back, turn where no road is, or lead to dead end, the route for given destination should be immediately invalidated. Car should switch to old random mode (brownian motion)? Also any road deletion or change of oneway/twoway, should invalidate routes stored at that tile. Recalculation of route should not be immediate, as the roadworks may still be in progress.
(Or the car should be destroyed in an accident for obeying the navigation without applying common sense).
Funny places in stephenson-siemnes-game: (448,108), (553,98), (392, 119), (884, 475)