Unsuitable for motor vehicles...

[Russell]

Does not mean no motor vehicles judging by the deep ruts in the mud, but it does make it unsuitable for tandems.   

[rafletcher]

Usually means ok for farm vehicles.

[MattH]

And can mean legal green lane for anything capable of dealing with mud and ruts - just not suitable for the average family car.

[pcolbeck]

And can mean legal green lane for anything capable of dealing with mud and ruts - just not suitable for the average family car.

Yup there is one on the edge of the village. The local green lane motorbike club come round about twice a year to ride it. Something about keeping them open by using them I think. Mainly used by horse riders, dog walkers and tractors.

[Paul H]

Does not mean no motor vehicles judging by the deep ruts in the mud, but it does make it unsuitable for tandems.   

If it's a white on blue sign it's advisory, they might absolve the council of some responsibility if you don't follow the advice. but they don't mean motor vehicles are not permitted, there's a different sign for that.

[grams]

Some of them are tarmacced but just a bit steep. Presumably to scare away sat navs and the unsuspecting.

[Ian H]

Around here at least it is a bit random whether there's an 'Unsuitable...' sign or an 'Unmetalled Road' sign.  Sometimes there's neither, just a small (4" dia approx) sign reading 'Devon County Road'.  At least one unmetalled un-tarmacked road near here has the statutory no motor vehicles sign with car & motorbike leaping over each other.

[FifeingEejit]

Round here this road is now signed as "Unsuitable for motor vehicles", below the starting position I've provided in GSV it is known to be owned by the quarry (Bredon) down to the A914.
From above the quarry to the adopted road at the top of Logie Brae, the owner has been completely untraceable despite both the council and the quarry trying to find out.

https://goo.gl/maps/rw5oWxtk8Z1KvK6k7

It wasn't too rutted when the GSV car went up in 2008, but you'd be lucky to ride it on an MTB without snapping forks now.