if a bridge is a marked as 15’6 just for example, and you trailer is 15’6, does that mean you can get under that bridge or not? It’s something I’ve never done just because it seems a bit too close for comfort
I would think its at the lowest part but I wouldn’t like to chance it - having said that many years ago I got lost and hit a bridge that was almost my height dropped all the suspension and crawled under it, it looked like I had more room than I should have but its hard to judge!
Yes there will be clearance but can never be sure by how much in practice, i’ve been under hundreds at exactly the height posted and never had an issue.
But you should know your vehicle and check the trailer height for yourself coupled to the tractor in its highest state if unsure for any reason…ie a Scania (other makes can raise slightly too, my MAN goes up about an inch) with the mid or tag lifted will likely be around 3" higher than with the axle down, if the trailer has a long overhang out front that could mean a 4" higher trailer at the leading edge…then if some clot drops the trailer suspension and fails to re-inflate it after will push the nose up another inch or two higher still.
Some trailers have a second height label on the front, height from rubbing plate to top of trailer edge, so handy to know your fifth wheel height mid lift up or down.
Its those who don’t think and don’t keep their wits about them that bash bridges.
Yeah, as I say I never chance it. But it’s got me wondering whether it means the marked height is the maximum height that CAN go under or does it mean that you have to be lower than that to get under. Ie on a 15’6 bridge you need to be 15’5 or less to go through.
My understanding is that if the warning sign is TRIANGULAR then this is advising you of the actual height of the bridge so at 15’ 6" a vehicle at no more than 15’ 6" will fit.
If the sign is a ROUND sign then it isn’t telling you the height of the bridge, it is telling you the height restriction beyond that sign and not the actual height of the bridge. However the bridge should not be lower than the height restriction but are often higher.
So basically - if the signs are 15’ 6" a vehicle at or under 15’ 6" will be OK - unless the road has been resurfaced or signs are wrong 
@Roymundo recently pointed the following out to me.
It would appear from the following manual that; they measure the exact height of the bridge, reduce by 3" to create a safety margin then round down to the next 3" increment. So if bridge measures 15’ 1" it would be signed 14 ’ 9"
gov.uk/government/uploads/s … ter-04.pdf
see page 31 for full details
But of course that does not mean to say that a 14’ 6" truck may not hit the above bridge - I always slowed down to walking pace if I was in a couple of foot, especially the arched bridges on undulating roads, would even stop and get out and take a look-see.
Was told that British rail use 13 inches to a foot might be true , it might not.
I put a 13.1 under a 13 foot and still had a good couple of inches left.
Was also told if the sign is circular it is law , meaning you can get under it but it’s illegal, bit like a speed limit- you can go faster but risk 3 points.
I know exactly to the half inch how high my unit & trailer is, if it’s a arch bridge the bit in the middle is always going to be higher but the height is marked at the lowest point & having a bit of local knowledge helps
Which helps me get a 12’9" trailer under a 11’6" arch bridge…it is a squeeze but saves me atleast 30 mins on a regular job
Just remember that the sign gives the maximum height of trailer allowed, not the actual height of the bridge.
Henk:
Just remember that the sign gives the maximum height of trailer allowed, not the actual height of the bridge.
Depends entirely on the shape of the sign
The way I was told about bridge heights is that the sign is the height of vehicle that can safely pass under the bridge.
I have taken a high cube container running at 14’ 6’’ under a 14’ bridge but I wouldn’t recommend it.
Get it measured. Scanias axle rides higher than DAF’s for example. Personally I wouldn’t risk it. I generally run at about 14’ but set the truck nav to 14’ 4". Only occasionally will I go under lower i.e along the A48 to Chepstow at 14’ 3" bridge heights.
I wouldn’t risk an inch or two but that’s just me.
Don’t be one of these T I T S that hits a bridge.
A tape measure is about £2 and can save save you points, money and job. It’s a no brainer really 
Why on earth would anybody see a bridge height sign, and knowing full well that their vehicle is higher than the allowed limit, proceed?
Are people that desperate to get home, or finish the job or whatever to risk wedging themselves under a bridge? If you have made an error with your route planning, simply re-route and get on with it.
[zb]ing numpties. Lorry drivers will never be seen as anything other than garbage while these tools are allowed to attend LGV steering wheels.
eagerbeaver:
Why on earth would anybody see a bridge height sign, and knowing full well that their vehicle is higher than the allowed limit, proceed?
And equally as worrying, when they know their height is lower than the bridge
It’s also worth remembering that a vehicle which might squeeze under a bridge when it’s laden may not do so when it’s empty. Less chance of that these days now that self-levelling air suspension is so common, but still worth bearing it in mind.
Good point valve. There are other circumstances also; You may have your mid lift axle up if running empty.
Also, a recently re surfaced road may have an effect.
My lorry is 13’11" empty after measuring many times to make sure. There is a 14" bridge near a yard I go to and I rather take a 3min detour to avoid it then find out it doesn’t fit 