I am doing a two tier this weekend for a birthday.  It is a 6 inch on top and a 9 inch on bottom.  It is done with buttercream, three layers on each tier so it will be pretty heavy top tier.  I normally use three dowels stuck in the bottom tier and just stack the top tier with the cardboard under it on top.  However, I have never had to travel with it already assembled.  Should it hold up if I drive carefully?  I don't think the top layer will slide because I am good at getting the dowels just even with the top of the cake so the top layer would be partly resting on the icing and then on the dowels, if that makes sense.  I am so nervous I am going to ruin this beautiful cake in the transport!!  

Delivering cakes is always nerve wracking! LOL

I tend to differ some from K8s remarks.  I have always put dowels around a circle 1/2"-1" less than the size of the tier going on top.  In this case it would be a 6" circle.  Yes, the dowels should all match each other in length.  Look for the highest spot on your cake and insert one dowel, mark, then remove and cut all the dowels to that height.

Using a good crusting b'cream, after doweling, put the top tier's lower border on, finishing the cake the night before so the border's b'cream can crust well. Store in refrig overnight (or, in the coldest place in/around/outside your home).  This usually will be enough to hold the top tier in place.

In the car,  place the boxed cake on a level place - the back floor or in the trunk. It would help if you had a piece of foam to put it on, but it is not totally necessary as long as it is *level*.

Drive like a 'little ole lady' :)

AIf you use SPS you wouldn't have a worry. I always drove normally. Hubby always drove like a man when we delivered cakes.

Whenever I deliver cakes, I always have someone holding it for me and making sure every things okay when I go over bumps and stuff. No matter what tier... call me paranoid lol. Actually the first time i delivered a cake without anyone helping me, the top tier sort of slid around, no joke. But it was so minute no one could tell, thank God! Only me 'cuz i Made it! From that point on, I never deliver alone. Too scared lol