The only Roman construction in Britain that basically vanished were the bridges, which were at one time pretty numerous. They fell into disrepair, but also were pulled apart for building stone because, hell I don’t need a bridge right now, and the next nearest source of stones is a mile from here. :’)
The old Roman roads used to plunge straight downhill, hit the bridge or ford, and keep on going, because the Romans were building roads to move troops, and they were on foot or horseback. Donkeys were also used to haul stuff, and the wagons and artillery (what we’d call catapaults and whatnot) were well made and had something like hand brakes installed.
In post-Roman Britain, the other reason the bridges weren’t kept up was that the barrows, carts, and wagons and such used to haul stuff couldn’t go straight down to the crossing. They had to take a slower, more deliberate, indirect route, which in British slang came to be known as a “birdfoot”. And the crossings were therefore moved.
After the Angles, Saxons, and Jutes invaded, the island was cut up into a couple dozen small kingdoms, some British, some newcomer. The Heptarchy (uh-oh, I’m not gonna look this up, and haven’t thought about this in some time, so here goes) was Wessex, Essex, Sussex, Northumbria, Mercia, East Anglia, and Kent (that was the Jutish kingdom). Welsh kingdoms included Dyved and, oh, I forget. Rheged was north of the Wall, and had the legendary King Coel (”was a merry old soul”). Cornwall was the Cornish kingdom (the Cornish language finally died out with its last speaker about a hundred years ago). There were others; Middlesex including London and environs, and was analogous to Washington DC, not belonging to any of the other kingdoms, but not being a full-fledged kingdom in its own right.
Commercial movement inside and between the kingdoms was on old Roman roads. Some of the major routes in use today were laid down right over the old Roman originals, which tend to run straight.
Now I'm going to go re-read "1066 And All That"....... :)
It really is amazing what the Romans built that is STILL standing today; their roads are one of the more famous examples and (because we’re on the subject of roads) there are some ancient New-World roads which extend virtually laser-straight for miles and nobody knows how they were able to do it.
The English roads are very good and do not suffer the constant pot hole problem that we do in Wisconsin. That is because they are built over a very thick base that starts with the Roman road. I don’t think that this is any road in the US that holds up so well. Gotta keep those road builders employed, so US roads are built on a planned obsolesence plan.