I can think of 2 options.
1. Carve the tiers
2. Add on to the side of each tier (extra cake or rice krispy treats) I think carving would be easiest.
When I made mine (in my photos) I wish I had taken pictures during the process. I'll try to explain the best I can.
1. Bake and stack each two layer cake as separate cakes on their own cake circles.
2. Start with the 12" tier. Put the 10" tier on top and mark the circle with toothpicks.
3. Lift off the 10" cake and set aside.
4. Then marking off the cake for the spiral cut, the top starting point will be directly above the bottom end point. Start at the top edge and gradually work your way down going all the way around the cake with your toothpicks. Mine looked like a funny pin-cushion with all the toothpicks.
5. Using a small serrated/steak knife, start at the top toothpick and spiral all the way down following your toothpicks until you get to the bottom. You'll need to cut far enough in so that when you make your vertical cuts (cutting out the 10" circle), your cuts meet.
6.After the spiral is cut, cut down around the 10" circle.
7.There you have something that looks all wrong, but nevertheless it is right.
8.Repeat these steps until you get to the top.
9.Next, I crumb-coated in the same green that I used for the grass. I didn't crumb coat the road.
10. Dowel (I used the Wilton wooden dowels) and stack with the start/stop points of each tier lined up in the back. Getting it lined up correctly is very important.
11. Ice the road before you do the grass.
You can make your finish line go all the way to the top, but my 4" cake was a little crumbly so I decided to quit while I was ahead. If any of these instructions are difficult to understand, feel free to ask more questions. It it kind of an illusion, but it is easier once you start it.
I will need your help unless someone else has an easier way?