You will need to provide a barrier that prevents the trucks from leaving the arena.  There are two methods you may use to achieve this.

1.) Place ground boxes all around the outside of the arena area.  These should be flush with the ground and extend at least seventy feet into the air.  Don't worry; they will not be visible in the game.  Set the first one up, then use copy and paste until you've got the track area completely surrounded.  You will probably want to turn off the Show Backdrop option so that you can see what you are doing.

2.) Place unapplied object boxes around the outside of the arena area (the same kind of objects you place at the base of checkpoints).  This is much more tricky to get into place, but has the advantage of minimizing lag in the game.  For most accurate placement, I suggest using 576x2x2 for the arena2.bin.  Turning on the wire frame will help you set it exactly at the wall's edge.  Then, add the seventy feet or so once it's in place.  For example, 576x2x70.  You will need four objects - one for each wall.  And, again, you will probably want to turn off the Show Backdrop option so that you can see what you are doing.

You do not need to use both methods.