I dont understand why some bridges are circle warnings but others are triangular.like the one under the A1 just past the m25.Because its in a triangle ,if you are only a lit bit over,you can try and squeeze under it?
Triangle signs are a Warning, (warning triangle)
Circle signs are an ORDER. 
Well, now we have AI, here’s my take. I’ve asked one of them chatbot thingys to explain the differences and this, in a nutshell, is what it came up with:
height warning signs inform drivers of low clearances ahead and allow them to make appropriate decisions, whereas height prohibition signs prohibit vehicles above a certain height from proceeding beyond a specified point.
Whilst this is undoubtedly correct and is the usual sort of standard answer to this thorny question, it doesn’t adequately explain the effect the different signs are attempting to achieve, IMO - what is the difference?
So I asked it to explain and received this reply:
Height prohibition signs are used when there is a specific reason to restrict access to vehicles above a certain height beyond a particular point (My emphasis). These restrictions can arise due to a variety of factors, including structural limitations, safety concerns, or the need to protect certain areas or assets.
Still unsatisfactory. So, in my inexpert conclusion, I assume the more severe sign is in place to protect a more important or vulnerable piece of infrastructure and the consequences of failing to comply are consequently more severe, though the financial (and other) costs incurred in a crash depend on the damage caused to the particular (usually) bridge.
You should take extra special notice of and care to avoid contravening the prohibition sign.
Oh,I see what you mean,the dartford bridge has a circle warning
Triangular signs are generally used on arched bridges, presumably because some loads greater than the posted height (e.g. roof trusses) can pass beneath.
I thought it was the following reason.
Circle means order, telling you height restriction ahead and you should not pass this sign or its an offense if you are over that height as you are ignoring an order. Also there may be no ways of turning around if you pass the sign and are over height.
Triangle means warning, there is a low bridge ahead, but you can continue if you wish as there are probably places you can turn around if you are over the height. You may have to go to a delivery point before the bridge, but its just warning you of the bridge.
I may be wrong but Im sure I was told this on a CPC.
Sploom:
I dont understand why some bridges are circle warnings but others are triangular.
When’s your next spot of DCPC due? Pick one that deals with Bridge Strikes. Triangle (warning) signs are associated with arched bridges, circular (prohibitory) signs are associated with flat or skewed bridges.
Driver hits a bridge and they’ll be going to see the TC (deffo no tea or biscuits), the typical HGV entitlement suspension these days is now six months - yeah, it’s that much of a big deal
Company will get their butts kicked at a PI too
Sploom:
Because its in a triangle ,if you are only a lit bit over,you can try and squeeze under it?
Who’s going to pay your bills when you’re out of work?
If only we had a dcpc trainer extraordinaire on here to explain it to us properly 
You need to play a recording of the mating call of the greater-crested switchlogic, I’m fairly confident he’ll have made one of those trendy vids covering the topic. Not being a huge fan of social media, I neither tik nor tok
Zac_A:
When’s your next spot of DCPC due? Pick one that deals with Bridge Strikes. Triangle (warning) signs are associated with
Sadly…
The DCPC does nothing at all to stop this and this is the whole argument against it. Because, here’s a driver that isn’t quite sure of the difference’s in signage that directly affect his journey yet, he’s passed the DCPC. I can’t think of a better example of failure than that.
Come on Switchy.
yourhavingalarf:
Zac_A:
When’s your next spot of DCPC due? Pick one that deals with Bridge Strikes. Triangle (warning) signs are associated withSadly…
The DCPC does nothing at all to stop this and this is the whole argument against it. Because, here’s a driver that isn’t quite sure of the difference’s in signage that directly affect his journey yet, he’s passed the DCPC. I can’t think of a better example of failure than that.
Come on Switchy.
You need to differentiate between the two types of DCPC
Sploom hasn’t passed periodic DCPC, because there aren’t any exams, I never did initial DCPC but I’m betting it doesn’t tell you everything you need to know, including bridge strikes, I don’t know if Sploom has done initial DCPC, I’m thinking he might have done as I don’t think he’s been driving since before 2009.
Some DCPC providers are f’in rubbish, others try much harder. But, hardly anyone shops around for quality/content, they just grab the first one they see, thinking they’re all the same.
I have a number of drivers as part of my TM responsibilities, there’s only one place in the Tyne-Tees region I will send them to for their yearly dose of DCPC, because I know they’ll get a useful day and won’t be bored witless. I attend the DCPC with them, so I don’t want to be bored witless either
The circles are probably on bridges of main train lines, whilst the triangles are probably on bridges of disused lines, so a bit less important if hit. That’d be my guess.
^^^ don’t ever pick my lottery numbers please.
the maoster:
^^^ don’t ever pick my lottery numbers please.
Why is he guessing? The answer is further back up the page
ezydriver:
The circles are probably on bridges of main train lines, whilst the triangles are probably on bridges of disused lines, so a bit less important if hit. That’d be my guess.
You maybe should have used pictures Zac, not big words! 
Zac_A:
Triangle (warning) signs are associated with arched bridges
Beau Nydel:
You maybe should have used pictures Zac, not big words!
A specific example
This one strikes me as odd, in that there are no markings on the bridge itself, and triangle signs on posts instead.
Ive only just done another cpc last month and it was about bridge strikes!
It never occured to me to ask the question then but its not a big issue because I simply treat the circle or triangle the same.I simply wont go under any of them.Thats my default position even if another driver has told me it fits
njl:
This one strikes me as odd, in that there are no markings on the bridge itself, and triangle signs on posts instead.
I would imagine in that case ^^^^ that it’s because of the position of the bridge on the bend. As it is with signs on the posts an approaching driver driving an overheight vehicle has advanced warning and can take a right turn prior to the bridge.
I’d also imagine that from the other direction the signs are in fact on the bridge.
ezydriver:
The circles are probably on bridges of main train lines, whilst the triangles are probably on bridges of disused lines, so a bit less important if hit. That’d be my guess.
Really!!! Have to ask where this idea comes from?
Although if we read the Highway Code it does say what they mean …. Triangle is a warning whilst circular is a prohibitive one