A lot of it did survive during the Byzantine era and big industrial projects such as harbours and bridges were still being built. People would have kept it and maintained it but the black plague and famine destroyed the population and economy. These ancient megaprojects need a large population to maintain and a lot of money, neither of which was available.
Fix: Calls made for 1500 streets to revert to 30mph.
Roads are for cars, streets are for people.