Heres a question… If the gross weight of my artic is 15 tonnes unladen, can I go down a road with a 17 Tonne limit ?
No. The weight limit usually means GVW and that will be more than 17 tonnes for your artic.
No, if your artic weighs 15t empty thats the unladen weight not the gross weight.
The gross weight is the maximum allowable weight and thats the weight that the limit applies to…
does it depend on the way the sign is written?
There are a few variations, some say MGW others might say “weak bridge 10t limit”
I mean some are there to stop lorries using an unsuitable road ie. damaging buildings etc or to stop people using a residential street as a shortcut, which is obviously a MGW sign.
If it’s for a weak bridge though and it has a 17t limit, then surely this means that anything over 17t is likely to kill the bridge and so long as you’re less than that it’s ok?
Some are stated as axle weights, how would you work that out?
I saw a sign on the A414 (i think) in hert’s between St Albans and Hatfield it read “weak bridge 12t”, then underneath “except buses” well if a 20t coach can go over it’s obviously not that weak is it! 
If it’s for a weak bridge though and it has a 17t limit, then surely this means that anything over 17t is likely to kill the bridge and so long as you’re less than that it’s ok?
The bridge would probably not object, but if plod see you going over it then you are done for… … …
Went over an old farm bridge (sort of made from Railway sleepers, bailer twine and pallets) with a Tesco van once, had someone training with me. They said sounding concerned “is that going to be ok?”, i replied “yeah…” which brought a look of relief, to which i added “…so long as we’re quick”
Gross Vehicle Weight, Maximum Gross Weight, and the new one, Maximum Authourised Mass, all mean the same. Whatever the vehicle is plated at.
The exemption for coaches and buses is on the condition that everyone aboard leaps out of their seats at the appropriate moment. 
Except for the driver. Actually, I think I’m right in saying that the PCV industry adopted ‘road friendly’ (air) suspension earlier than did the truck industry, hence, room for a degree of tolerance.
Axle weights. Just work on the basis that the maximum weight for an axle is 10t, so unless you know the weight to be less. Don’t use it.
dennisw1:
does it depend on the way the sign is written?
Yes, some refer to actual weight as opposed to GVW / MAM. Not sure how common they are though.
Actually, I think I’m right in saying that the PCV industry adopted ‘road friendly’ (air) suspension earlier than did the truck industry, hence, room for a degree of tolerance.
I know what you’re saying, but in the example i quoted a 20t coach weighs 20t no matter what suspension it has