Time: Roman Empire
Terms searched: ox, bullock and variants with "cart" "wagon" and "transport", and "Great Trunk Road", "Grand Trunk Road". Result: a lot of information on oxen on farms and the route of the Grand Trunk Road.
Books searched: my extensive collection dealing with the Roman army and Roman roads. Result: information on the construction of the paved road and the road network of the Roman Empire.
I have come across two references - one in Rudyard Kipling's "Kim" (which I originally discarded because I didn't think it would be relevant to Europe) and one in Dornford Yates' "Jonah & Co" (France, 1920s) - relating to soft tracks being constructed alongside paved roads for slow-moving ox-carts to use without holding up faster traffic. The more I think about it, the more logical and sensible it sounds, because it would be kinder to the hooves of cattle, as well as the fact that such carts could be mind-numbingly slow.
What I do know about Roman roads, apart from the manner of construction of the paved part, is that there was extensive space to the side of them, promarily to safeguard against ambush. In addition, though carts might get bogged down, the wheels of the time on roads made with paving stones for long distances would jolt unbearably. Given that baggage trains were ox drawn, and that above all nothing should interfere with Roman soldiers marching along the roads or couriers riding along them, it seems logical that the Romans would have done the same - but logic is not enough. Has anyone come across any suggestion that the Romans did have this sort of provision for slow-moving carts?