Are you saying that the ruts were pre carved and constructed. A designed and constructed rut-way?
If not how many vehicles would have to pass over the exact same spot to produce the ruts in the first place, each placing their wheels in exactly the same spot as the previous vehicle, until a discernible rut appeared.
What caused the vehicles to follow exactly the same route time and time again?
How were they navigated prior to the development of the ruts without defined roadways or choke points??
Why do we see central depressions in only a very few places, yet rut tracks in 100% of places?
Even if the loaded journey was downhill the empty vehicle would need to make an uphill return journey.