Live and Learn. I lived through it and learned that I should never attempt to transport a three tier cake. I spent 5 days baking, making icing, crumb coating, chilling, frosting and decorating a square (8", 12", 16") three tier cake for a wedding an hours drive away. It was doweled throughout on all layers and also one center one all the way through. When I arrived at the venue and opened the tailgate I was horrified to see that the cake had totally fallen to one side. Luckily I had brought along extra icing, cake tips and pastry bag for any little mishap that could happen. (When my DH put it in the back of the car his hand slipped off the cake board and all 4 fingers when right into the bottom tier. So I had made one repair before ever leaving the garage.) However, this was much more dramatic. After being revived I went to work cutting off the most damaged sections, piecing the two bottom tiers together to make one complete layer, repairing the top layer and putting it on the bottom (this is now a 2 tier cake) and spent 2 hrs. re-doing, re-frosting, re-piping designs to make it presentable. All while being watched by the entire wedding party and various other relatives. And their photographer got it all in pics. How wonderful (sarcasm). The good news....I managed to hold back my tears, there was still plenty of cake to go around , AND it didn't rain on the outdoor ceremony. Good thing I wasn't having them pay me for it - it was my gift to them.
Sorry you had such a disaster, but you held it together and managed to repair it...with an audience, even ~ which would make most of us shake in our boots!!! They say the true test of a great decorator is how they perform under pressure, and it sounds like you passed with flying colors! Chin up! The worst has happened and you got through it.....relax now, and figure out what you will do differently in the future....be it assemble on site, use the SPS system, whatever.....
I don't know HOW you managed!! I had a cake that did the same thing a year ago. I held it together like you, but they only had their top tier and a kitchen cake to show for their wedding. At least you were able to still present a cake. I'm so sorry this happened.
I will NEVEr use wooden dowels again (not sure what you used) and only use sps stacking system now. I charge the brides a rental and deposit on them that covers the replacement several times over (because I don't like to have to order more lol) and I never travel with them assembled (even though everyone says it's ok to do). Some caketastrophes are just so hard to get over, especially when it's for someone's wedding. I totally feel your pain on this one, but you did an absolutely wonderful job!! I can hardly assemble a cake with no issues with a photographer watching and snapping photos......I can just imagine how you felt having to piece together a cake with everyone watching!!! You poor thing!!1
Don't let this discourage you, learn from it and make the necessary adjustments in how you operate and move on!! You can only go up from here!
The good news is that you only have to experience your "first" disaster just once. After that, it's called experience! 
dont' let it discourage you from transporting a pre-assembled cake. It can work. for one an hour away, I probably would have transported unassembled and put it together when I got there, but cakes are transported assembled all the time. Dont' give up on it!
(and warn hubby to watch his fingers! If he wants to lick some icing, he can have the bowl after you're done!) 
I recently took a cake 45 miles from here and did carry it un assembled. When I go there the two smaller cakes shifted on their boards some, so I was glad I didn't try to stack first. I think for long distances it is probably better to plan to stack when you get there, but I have carried stacked cakes shorter distances with out issues. 
I'm sorry this happened to you, but kudos on the repair job.
I had a similar disaster earlier this year. Mine was caused by the heat I lost the entire grooms cake. had to rush to Walmart buy sheet cakes, icing and strawberries ($102) to make a grooms cake in 45 min. Nerve racking! My sympathies are with you. Good luck on your next stacked cake. I use wooden dowels also and mine always travel a minimum 45 min, that was the first time I had a problem.
Seriously, guys and gals, using SPS you can safely transport a stacked cake. As IndyDebi has said before, if the cake will survive a 5 mile trip, it will survive a 50 mile trip.
Dowels are not a good support system and the center dowel is false security. The dowel will stay firmly embedded in the bottom board, but has little effect on keeping the cake stacked.
SPS. Repeat after me, SPS.
In addition to the SPS, the wedding cake boxes made by Bakery Craft and sold at GSA really help! We used both at the grocery store bakery where I used to work and (true story) I swear it is only due to the two together that this cake held together:
We were short handed and had a Sunday wedding cake delivery so one of the grocery managers was to deliver it instead of any of the bakery staff. He put the cake in the back of his pick up truck in the front part of the bed next to the cab. He then roared out of the parking lot and up street that was a rather steep hill. The cake box of course whizzed down the length of the truck and slammed into the tailgate. Some how, when he got to the wedding and pulled the cake out it was still perfect. This was an all BC cake, too. It was either a miracle or the combined powers of the SPS and those cake boxes. This manager was never allowed to deliver another cake, though. We weren't going to chance fate twice.
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