Yeah, your use of Autoruns and Parallels here is going to be disastrous.
Autorun events will continually run and block everything else from working until they are turned off. Parallel Process events will continually run (meaning they will keep looping back to the top every time they've evaluated every event command - which is why you're seeing the behavior Andar described) while play occurs, and tend to cause lag when used excessively.
When you're trying to set movement to non-interactive objects like these cars, you're best off putting the Move Routes in a separate event and choosing the cars as the target of the Move Route. Make the cars into Action Button events with no code. And if you want the car's Move Route to keep repeating rather than just happening once, then choose the "Repeat" option in Move Route. If you want that separate event to be an Autorun, be sure to Erase it or turn it to a non-Autorun page after completing the "Yes or No" branch.