Where a vehicle deviates from a calculated optimal route, the system displays a message indicating the vehicle's deviation. The driver of the vehicle commands computation of a new optimum route. The system selects a new starting point from a point substantially ahead of the vehicle, recomputes the optimal...http://www.google.de/patents/US5262775?utm_source=gb-gplus-sharePatent US5262775 - Navigation system with off-route detection and route recalculation
