Reading the posts on here, I’m apparently not the only one who is puzzled at the apparent differential in tolerances on bridge heights. For example, you pass under two bridges both marked at say, 13’6", the first nearly takes the roof off your 13’5" trailer and the second you could go under with a foot to spare, or so it seems.
Like many of you I’ve always put it down to re-surfacing, subsidence or whatever, but a chance conversation on another forum with a retired highways engineer has enlightened me as to how they’re actually measured.
Apparently the formula is to measure the bridge, subtract three inches, then round the figure down to the nearest multiple of three inches if it happens not to be exact. For example, a bridge measured at 15’ 2" would have a warning sign for 14’ 9", being reduced first to 14’ 11" then rounded down to 14’ 9".
It follows that if the bridge is dead on 15 feet, the bridge would still have the same height marker, and as our wives are fond of telling us, that extra two inches can make a huge difference!
It would also follow from this formula that a trailer should always be able to pass under a bridge marked at the same height, provided of course that both have been accurately measured. Whether this takes into account the extra length of modern trailers when passing under a bridge situated in a dip in the road, I do not know.
Hope this might help someone; they do say you never stop learning in this game. I would add though that it’s not an invitation for any more daring or stupid members to test the boundaries of this formula; if you go under any bridge with a trailer marked over the bridge’s height you deserve all you get if it goes mammaries skywards.
Reminds me of that one under Derby ring road, nearly caught me out years ago when I approached in a Transit Luton instead of me normal panel van; just remembered in time!
Sidevalve:
Reading the posts on here, I’m apparently not the only one who is puzzled at the apparent differential in tolerances on bridge heights. For example, you pass under two bridges both marked at say, 13’6", the first nearly takes the roof off your 13’5" trailer and the second you could go under with a foot to spare, or so it seems.
Like many of you I’ve always put it down to re-surfacing, subsidence or whatever, but a chance conversation on another forum with a retired highways engineer has enlightened me as to how they’re actually measured.
Apparently the formula is to measure the bridge, subtract three inches, then round the figure down to the nearest multiple of three inches if it happens not to be exact. For example, a bridge measured at 15’ 2" would have a warning sign for 14’ 9", being reduced first to 14’ 11" then rounded down to 14’ 9".
It follows that if the bridge is dead on 15 feet, the bridge would still have the same height marker, and as our wives are fond of telling us, that extra two inches can make a huge difference!
It would also follow from this formula that a trailer should always be able to pass under a bridge marked at the same height, provided of course that both have been accurately measured. Whether this takes into account the extra length of modern trailers when passing under a bridge situated in a dip in the road, I do not know.
Hope this might help someone; they do say you never stop learning in this game. I would add though that it’s not an invitation for any more daring or stupid members to test the boundaries of this formula; if you go under any bridge with a trailer marked over the bridge’s height you deserve all you get if it goes mammaries skywards.
So, did you also find out why some bridges have a warning sign and some have a prohibitive sign?
Sidevalve:
Reading the posts on here, I’m apparently not the only one who is puzzled at the apparent differential in tolerances on bridge heights. For example, you pass under two bridges both marked at say, 13’6", the first nearly takes the roof off your 13’5" trailer and the second you could go under with a foot to spare, or so it seems.
Like many of you I’ve always put it down to re-surfacing, subsidence or whatever, but a chance conversation on another forum with a retired highways engineer has enlightened me as to how they’re actually measured.
Apparently the formula is to measure the bridge, subtract three inches, then round the figure down to the nearest multiple of three inches if it happens not to be exact. For example, a bridge measured at 15’ 2" would have a warning sign for 14’ 9", being reduced first to 14’ 11" then rounded down to 14’ 9".
It follows that if the bridge is dead on 15 feet, the bridge would still have the same height marker, and as our wives are fond of telling us, that extra two inches can make a huge difference!
It would also follow from this formula that a trailer should always be able to pass under a bridge marked at the same height, provided of course that both have been accurately measured. Whether this takes into account the extra length of modern trailers when passing under a bridge situated in a dip in the road, I do not know.
Hope this might help someone; they do say you never stop learning in this game. I would add though that it’s not an invitation for any more daring or stupid members to test the boundaries of this formula; if you go under any bridge with a trailer marked over the bridge’s height you deserve all you get if it goes mammaries skywards.
You don’t need to work all that out, its already been done for you. You’ll come across either a warning triangle or a round prohibition sign. Are you suggesting people take the hard route and do their own sums to get under a bridge rather than obey the signs that are in place?
Mike-C:
You don’t need to work all that out, its already been done for you. You’ll come across either a warning triangle or a round prohibition sign. Are you suggesting people take the hard route and do their own sums to get under a bridge rather than obey the signs that are in place?
Thank you Mr. Grumpy, I’m quite aware of that. I simply thought that members of this forum might be interested in a bit of the “science” that goes into these things. If you’d taken the trouble to read my last statement you’'d have answered your own question.
Wilton bridge on the A36, nr Salisbury is marked at 14 ft. I am driving a Ren Magnam, that is marked at 13 ft on the cab height.
As I approach S/bound, I am told “High vehicle. Use middle of road”
As I approach N/bound, it tells me " Over-height vehicle. Divert"
SWEDISH BLUE:
Wilton bridge on the A36, nr Salisbury is marked at 14 ft. I am driving a Ren Magnam, that is marked at 13 ft on the cab height.
As I approach S/bound, I am told “High vehicle. Use middle of road”
As I approach N/bound, it tells me " Over-height vehicle. Divert"
I used to go under a bridge marked 14’ with a trailer marked 14’ 6", I would never have attempted this till one day I watched an agency driver go under it (seems he had always gone that way ) and there was at least 6" of clearance and it didn’t trip the warning lights. I measured our trailers that had always been marked at 14’ 6", they were actually 13’ 11" and the bridge marked at 14’ was easily 14’ 6". Both measurements erred on the side of caution but I had done a 20 mile detour for years because of it
Approached a bridge once with a track machine on board. Not being sure of its height I brought it up close to the bridge and got out to check, a good foot of room so I proceeded slowly only to catch it 3/4 ways through!! Bridge was a foot lower on other side !
I knew about the bridge heights.
But I also thought trailers and wagons were also measured similarly. IE. Height rounded up to nearest 3".
That was until I got into a wagon that was marked 11’7". Confused, I measured it to find that empty it was 11’8 at the front, and 11’11" at the back. So I marked it up at 12’.
Fully loaded however, it was 11’6" front and back. So I am guessing it was loaded when it was marked up.
Many years back I remember this driver overshooting his delivery point, he went under the bridge to turn around up the road, came back under the bridge or should I say attempted to come back under the bridge, he got wedged and tipped the thing over…not good.
