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There's a trail near me which is great link to the hills, avoiding busy twisty narrow lanes. The second half is a byway, the first half is just marked with green dots on the OS map.
The OS map legend refers you to highways or the definitive map. I checked the definitive map and that's no different! Just dots (albeit red this time).
Reason for asking is its all used by 4x4s and bikes. The second half holds up fairly well but the first half is upon downy clay track alongside z stream - and is churned to knee deep slop, 3ft deep ruts or bottomless sinkholes by traffic.
Just wondering if its worth pursuing ang sort of reinstatement/repair, or even if the motorised use shouldn't be happening, getting this restricted maybe. But very much depends on use class - I wouldn't want to stop the 4x4s and bikes having their bit of fun as there's not many places they can do it.

This is a better bit, some is completely under water for 20 or 30m at a time

Those red dots mean an unclassified road. So most likely legal for motors.
Is there a minimum standard which an unclassified road should be maintained to?
think the best you'll get is seasonal restrictions (with a lockable gate at either end). We've had a couple put in locally, no vehicles between october and march, though it makes no difference as nearly all the damage is done by farm traffic.
Because the council has to keep roads, even unclassified ones, up to a usable condition they basically restrict them to non motorised traffic so they don't have to bother. This happened quite a few years ago to most of the Byways. This meant all the trail motorbikes and 4x4 users ended up with pretty much nowhere to play. This then concentrated their playing to even fewer areas, thereby making any problem worse. Subsequently the trail motorbike industry has died, anyone left is now having to play cheeky.
But, as mentioned above, most damage is farming and of course users who ignore restrictions. Personally I would not bother trying to get the council to restrict the track. Just ride round it or go somewhere else.