what if this was after TSHTF?

[Devonian]

The fact is that yes this will be true in some places, but in other places no.

For example the 2 bridges referred to are two very old stone bridges which were subjected to serious scour during one of the wettest winters on record, which undermined the bridges causing them to collapse.

There are however probably another 2000+ bridges in Devon which did not fail.

Also none of these bridge have realistically had any maintenance other than maybe the rebuilding of parapet walls from where vehicles have hit them, as for drainage, there is drainage on these bridges.

And most of these bridges have been standing for 100's of years, long before cars were around and way before the current population was at it's present levels, so why would they suddenly fail in the future???

As for the newer modern bridges, then look at structures like the Torridge Bridge or the Kenwith Viaduct or the South Molton Viaduct, with these there has been no maintenance on these structures since they were built in the 1980's, yes they are heavily trafficked but these sorts of structures are designed to be maintenance free for at least 30+ years.

The main routes will remain passable and useable for many years after any event, the worst affected roads will be the back roads where the hedges and verges start to swallow the roads.

The biggest issues will large trees coming down in storms which can't be easily cleared and rivers bursting their banks and washing out sections of roads, but in the "wider scale of things", these are like to far and few between.

I think it is far more likely that in any serious situation groups and towns will simply block roads by felling trees etc to provide added security and protection against others entering their area.