Might I point out, however, that you will go SLOWER than normal walking speed if you were forced to push a cart down a road?
Weeeelllll.... Slower than juggling twenty different kinds of items, like loose potatoes and bits of stone and wood while holding a weapon and shield at the same time? Constantly dropping things and unable to even see where you're stepping because all the crap you're carrying is in the way? It's like trying to carry several bags of groceries inside, only without the bags and without the front door just a few steps away from the car.
LOL, I'm just being difficult. Speed would be an acceptable sacrifice for traders as long as you could carry a lot more stuff; though it *would* be nice if there was also some sort of way to speed up a little without needing a big complicated vehicle.
Of course, if domesticated horses were introduced we wouldn't even be having this conversation... *cough*