I just transported a 3d Jeep cake without problems. I would say to make sure it is well stuck to the cake board and well dowelled. If you have any pieces sticking off it (like the bumpers, mirrors, and spare tire on my jeep) I would put in some external support just for the ride. The only other thing you can do is drive slow, careful and avoid as many bumps as you can!
How secure is you cake to your board? Will it slide off if you move it around? If the ckae will stay still and you put your cake on a flat, non-skid surface during transport, and drive real careful, you should get to where you are going with no problem. Try to determin if you cake will tip. You might need to support it somehow and take supplies with you to fix any damages. LOL 
There is a horrific story on one of the threads about a castle cake falling apart during the customer's ride home. She was advised to secure it with dowels. I can't seem to find it, but do a search. You may get some more tips.
As far as lessening the bumps in the road and the ride, do like the wedding cake delivery people do and cushion the box with that foam you can buy at lumber yards and hardware stores. I think you can actually line the bottom of the box with it to for extra comfort.
I use a teaspoon of royal icing under the cake to stick it to the board and then it won't move at all!
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