And so soon after my last one, too. Thankfully it wasn't anywhere near as bad as it could be, but I'm feeling the need to vent >_<;
The cake (http://cakecentral.com/gallery/2305308/playing-card-cake) was for my little brother's 17th birthday, and I'd been up until the early hours of the morning finishing it with two sore, bandaged fingers (gotta love those bread knives!), only to have not just one, but TWO different people pull out in front of us on the way to deliver it. Both of them came within inches of hitting us, and my other half had to slam the breaks on so hard *I* nearly hit the dashboard, let alone the poor cake. Luckily, the supports held up brilliantly, and I had it on my lap (with my legs at a funny angle so that it would sit flat and not slide), so I managed to rescue it. I'm amazed it made it in one piece. It's by no means perfect, but he was made up, so that's what matters to me <3
That is the problem with riding the cake on your lap. The safest place is in the trunk of your car. I use a rubber mat under my cakes and they do not slide. If your support system is good it will take the minor mishaps that you have delivering. If you have it on your lap you will tend to grab the cake or be pushed forward into the cake with a hard stop. Sorry this has happened to you, sometimes I wish I could have a flashing light on top of my car with cake on delivery.
Cashley, do you think a rubber mat would be good enough even with some hard braking? I've been thinking about investing in one, but as I've only really made cakes for family and friends so far (and nothing too large), I haven't looked into it properly.
As cashley said, the trunk is the safest place to have your cakes for delivery. You can go to the dollar store and get the rubber matting like you line shelves with and it will keep them from moving. I have 3 huge pieces of it in my vehicle and love it. I have a shelf that can be used across the back (suv) and have put large 1/2 cakes on it and nothing moved. Good luck!