Collision Trucks: 

1) the principle your toy is based on: The main principle that I wanted children to take away from this toy was of momentum (with a little bit extra of collisions - maybe elastic/inelastic). 

2) how will the child interact with the toy: The child will simply put in two trucks in the ends of the tube, set the speed for the trucks, and then watch them crash! The machine will then spit out the trucks at the end of the collision. 

3) construction cost, durability, safety: I think the construction cost will be in the $35-40 dollar range, as there will need to be one main rail, two motors, one main control board, and then aesthetic cover ups. Thus this toy will definitely be in the more expensive range (retailing at about $90-$100 dollars). Durability shouldn't really be an issue, besides for the toy cars, but toy cars are rather inexpensive anyway. Safety also shouldn't be an issue as as long as the main tube does not break and the child cannot stick their hands into the car tube, then no real accident could happen (unless the child hits another child with the tube). 

4) what I learned from the experience: I realize it is quite difficult to connect a principle to a toy in a way that is fun. When I look back and think of the toys that really taught me something, they were mostly Legos, map games, and it was honestly quite clever how much I learned and practiced in a way that I thought was fun. In this specific case, I think I learned that it may be a lot more difficult for the child to actually learn something from crashing trucks together (they may not pick up the idea of momentum), and might just do it thoughtlessly.