Never on a lap... always on a flat surface. The dollar store sells a rubber like mat roll with little holes in between the mesh that works really good;
place that on the floor of the car or the flat area of the car or van and then place the cake which is inside a bakery box on top. Never transport more than 2 layers stacked and make sure stacked layers are doweled well. Drive slow, allow enough time for whatever may happen. Keep the cakes cool...
If your icing cracked...your cake board probably flexed a little. I use triple thickness of board on the base cake...and put tiers on flat cookie sheet type pans so no flexing can occur. The rubber-like shelf liner that was mentioned is perfect to put between your cardboard cake base and the flat "pan" or whatever is the flat surface is that it sits on while traveling. Some people like to use the egg-crate mattress protectors under their cakes when traveling.... also, boxing your cakes/tiers protects them from moving around. Boxes are relatively cheap, and you can figure their cost in the total cost of your cake.
Definitely use at least double-boards, triple for extra large or stacked/tiered cakes. It's not worth all the time and effort that goes into decorating to have the icing crack because of a flexible board. I prefer to err on the side of caution 
Marilyn
I agree, if the icing cracked it because the cake got "bent" somewhere along the way. The hardest part of transporting a cake is moving it from the table to the car, then the car to the reception... If the cakes are not on plates then make sure you have enough cardboard under them to keep the cake from bending...
I have only transported a couple of cakes...see my gallery...
The ones that are separated, I put the cakes on the cake plates then I laid a damp towel on the floor of my suburban, put the cakes on the towel and drove SLOWLY. (I don't know why the damp towel works, but it does...those cakes didn't move!)
I have only transported one stacked cake, and I put the whole thing together (3 layers) with support dowels on the bottom two layers, and one dowel right down the center of all the layers. (Sharpen the end of the dowel and use a hammer to get through the cake boards...) That center dowel kept the cake from shifting and again drive SLOWLY. Then I just added the flowers at the reception.
Good Luck!!
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