In reply to pebbles:
I'm not sure about the specifics of this case, but it appears you might not be familiar with the general process of creating a diversion route. It probably wasn't "tumbleweed and fluff" running through their heads, they will have merely been following perfectly sensible regulations.
There are two constraints to creating a diversion route. The new route has to have enough capacity to support all of the extra traffic (the diversion can't use roads of a lower classification that the closed road, e.g. if you close a B road, you can't use a C road for the diversion) and it has to be accessible enough (wide enough/no tight bends/no weight limits/no steep hills etc.) that any traffic which would have gone down the closed road can make it around the diversion route.
If you're a car user with local knowledge (or a map) there will often be a more appropriate route, but that doesn't mean the diversion is incorrect.
(or at least this is my understanding, happy to be proved wrong)