Creating Boundries & effects

I am working on a program and i would like to be able to let the user control a vehicle on the program. The issue that i am having is that i would like to restrict the user to stay within a certain circle. This is what i am unsure on how to do.
Does anyone know how to go about creating random movements for a vehicle? I think this could be accomplished with having a variable that is randomly selected(forward, reverse, left, right, etc) then random choose a number for the distance to travel.
And going along with the second part, how would i write(if its even possible) that if the user controlling the car, collides with the other car(computer controlled), then do a method(crash sound, fire, etc)?

Any help would be much appriciated.

For the boundary, place an Object in the world, drop a Object move to vehicle tab into the world screen, and then move the Object to the center of where you want the boundary to be. Then, make a repeating if/else statement (True) that tests whether the vehicle is inside a threshold of that central Object. In the else box, put vehicle move one meter towards central Object. That should keep it in the boundary.
In case that was confusing, if you read what the program does aloud, it would say: If vehicle is within threshold of central Object, do nothing. If vehicle is not (else) then vehicle move one meter towards central Object.