Hi Paul and yes I know what you are saying and I'm regularly out and about in the darkest parts of the county. I guess a lot depends on what you mean by driveable??
Will we be able to cruise along at 40mph, then no, but pot holes, broken up tarmac and washed out sections do not necessarily make a road in passable or unusable - unless of course you are taking your wife out in the MX5 again
A lot of the UCR's are simply the old mud and stone pony tracks which have had thin layers of tar and chippings sprayed on over the years to provide the metalled surface.
As for the Tarka Line, then yes, this follows the route of the river on a flood plain, but I'm pretty sure the trains won't be running anyway.
Don't get me wrong I'm not saying there will not be problems, but the problems that do occur will either be passable or they will be isolated issues meaning alternatives routes will be used/found.....
As for roads flooding, yes they flood and may be impassable for a few weeks, but then it clears..... look at the Somerset levels, they haven't had to rebuild any road there despite them being under water for months....
With my Civil Engineering (hard) hat on I just don't believe that the entire transport network will just suddenly fail within a decade or 2........
Also look at the talk of reinstating the Exeter to Okehampton rail route, it hasn't been used for several decades, but most of it is still there and could be put back into use with little effort.