I haz duh solution.
It's safe to assume that half of the cars/trucks are going uphill when the other half is going downhill. Sooooooo, we put all of them on a cable system. The half going downhill will pull the half going uphill over the hill thus they become the energy source required to pull the next one up the hill. We will only need to add in just a little energy to cover for little old lady school teachers that only drive their cars to school where it is down hill both ways.
To hell with walking like an Egyptian, engineering like a DUmmie is more fun.