How accurate are distance signs?

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Having a cuppa
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How accurate are distance signs?

Post by Having a cuppa »

Engineering standards are currently in metric units, apart from speed which is fortunately still in miles per hour, and motorways are built to metric specifications. I skimmed through the TSM and while I couldn't find any information regarding regular signage, Chapter eight did state temporary signs should be placed at multiples of metres and signed in yards without converting as the 10% tolerance allows for it. For example 400 m would be signed as 400 yds, and 1600 m would be signed as 1 mile. I was also reading on this thread viewtopic.php?f=1&t=36508&start=20, distances rounded from metres to yards or miles aren't accurate.

So how accurate are the distances on signs?

And if a sign is placed at a 400 m distance, why not sign it as 440 yds or 1/4 mile, instead of 400 yards?

Does Highways England lack the skills to place a "1 mile" sign at 1 mile or at least accurately convert from metric?

At least 40 mph will always be 40 mph no matter what.
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Re: How accurate are distance signs?

Post by Chris5156 »

Having a cuppa wrote: Mon Feb 01, 2021 03:36 Engineering standards are currently in metric units, apart from speed which is fortunately still in miles per hour, and motorways are built to metric specifications. I skimmed through the TSM and while I couldn't find any information regarding regular signage, Chapter eight did state temporary signs should be placed at multiples of metres and signed in yards without converting as the 10% tolerance allows for it. For example 400 m would be signed as 400 yds, and 1600 m would be signed as 1 mile. I was also reading on this thread viewtopic.php?f=1&t=36508&start=20, distances rounded from metres to yards or miles aren't accurate.

So how accurate are the distances on signs?

And if a sign is placed at a 400 m distance, why not sign it as 440 yds or 1/4 mile, instead of 400 yards?

Does Highways England lack the skills to place a "1 mile" sign at 1 mile or at least accurately convert from metric?
It's not about skills. If you can measure 1600m and put a sign there, you could equally well measure a mile.

My question would be, what would we all gain if the 400 yard roadworks sign was moved 40 metres? Does it matter?
At least 40 mph will always be 40 mph no matter what.
Do you mean 64km/h? :stir:
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Re: How accurate are distance signs?

Post by Stevie D »

Having a cuppa wrote: Mon Feb 01, 2021 03:36 Engineering standards are currently in metric units, apart from speed which is fortunately still in miles per hour, and motorways are built to metric specifications. I skimmed through the TSM and while I couldn't find any information regarding regular signage, Chapter eight did state temporary signs should be placed at multiples of metres and signed in yards without converting as the 10% tolerance allows for it. For example 400 m would be signed as 400 yds, and 1600 m would be signed as 1 mile. I was also reading on this thread viewtopic.php?f=1&t=36508&start=20, distances rounded from metres to yards or miles aren't accurate.

So how accurate are the distances on signs?

And if a sign is placed at a 400 m distance, why not sign it as 440 yds or 1/4 mile, instead of 400 yards?

Does Highways England lack the skills to place a "1 mile" sign at 1 mile or at least accurately convert from metric?
When you're driving at motorway speeds, the difference between 1600m and 160932cm is irrelevant and not noticeable. Even the ~10% difference between 400m and 400yds is irrelevant and not noticeable. Given the limitations on where it is suitable to install signs in terms of visibility and terrain, there will inevitably be times when some tolerance to the official distance is needed, so we might as well go with the plan of "Pick the most sensible site that is roughly 400m away and call it 400m".

By keeping to round numbers rather than worrying about a trivial and unnecessary level of accuracy, they can keep the calculations and measurements simple and reduce the risk of mistakes (eg people adding 10% rather than taking away 10%). That isn't to say that the people involved can't do the sums, but that they are human and sometimes people make mistakes even when they know perfectly well what they are supposed to be doing.
At least 40 mph will always be 40 mph no matter what.
Or within the +0/-10% tolerance allowed on the speedometer 🤔
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Re: How accurate are distance signs?

Post by Debaser »

Guidance on the location of signs on motorways and all-purpose trunk roads used to be in IAN144 (I don't know which of the new DMRB standards it will now be in).
Exit Datum Point
2.2 An exit datum point (EDP) is a fixed point defined by the highway geometry, on which all sign and signal spacing is based. In generic terms, the EDP is located at the start of the taper to the diverge lane at junctions where there is no loss of lane through the junction. At lane drop junctions, except as stated in paragraph 2.3 below, the EDP is 200 metres in advance of the diverge tip of the nose markings. Where there is a double diverge, the EDP is positioned relative to the first diverge or lane drop.

2.3 At lane drop junctions, where a single lane drop widens to two lanes prior to the diverge tip of the nose markings, the final sign gantry should be located at the start of the taper where the carriageway widens from a one lane to a two lane drop. In some locations this point is closer than 200 metres from the diverge tip of the nose markings and the EDP would therefore still be located as stated in paragraph 2.2

Location of Final ADS
2.4 The final ADS shall be located at the EDP. If this cannot be achieved due to site constraints, then the final ADS shall be provided at a location between the EDP and 50 metres upstream of the EDP (IAN 111/09 section 10.11 paragraph 3 [Ref 8]).

Final ADS and 1/2 (1/3) Mile ADS
2.5 At grade separated junction diverges from the main carriageway on both motorway and all-purpose trunk roads, a final advance direction sign (final ADS) shall be provided together with a 1/2 mile (ADS) (TD 22/06 (DMRB 6.2.1), paragraph 5.36 and 5.37 [Ref 3]). Where there is inadequate space or there are other constraints such as the positions of bridges, the 1/2 mile ADS shall be replaced by a 1/3 mile ADS (refer to paragraphs 2.9 to 2.12 for tolerances on ADS locations).

1 Mile (2/3) ADS
2.6 At every motorway and some all-purpose road junction diverges from the main carriageway, a 1 mile ADS shall be provided (TD 22/06 (DMRB 6.2.1), paragraph 5.36 and 5.37 [Ref 3]). Where there is inadequate space or there are other constraints such as the positions of bridges, the 1 mile ADS shall be replaced by a 2/3 mile ADS (refer to paragraphs 2.9 to 2.12 for tolerances on ADS locations).

2.7 At grade separated junction diverges on all-purpose roads, a 1 mile ADS (2/3 mile where there is inadequate space or there are other constraints such as the positions of bridges) should be provided where the road has a speed limit of 50mph or more, and more than two lanes, or the designer feels it is necessary for other reasons such as volume of traffic or safety.

Confirmatory Direction Sign
2.8 A confirmatory direction sign shall be placed over the ‘diverge’ (gantry mounted signs) or on the nosing (verge mounted signs) (TD 22/06 (DMRB 6.2.1) paragraph 5.37 [Ref 3]). When the sign is mounted on a gantry, it shall be located between 30 metres and 50 metres downstream of the tip of the nose of the road marking (TD 46/05 (DMRB 9.1.1), [Ref 9]). Where a verge mounted ADS is used, the confirmatory sign takes the form of a rectangular direction sign, and shall be located on the ‘diverge’ island not more than 30 metres beyond the physical nose (TD 22/06 (DMRB 6.2.1) paragraph 5.37 [Ref 3]). If there is inadequate space to provide a verge mounted sign within the required distance, then mounting the sign on a cantilever gantry located on the nearside verge should be considered.

Tolerances on ADS Locations
2.9 Where the 1 mile and 1/2 mile ADS distances cannot be achieved due to site or construction constraints, then a tolerance of +10% increase / or 20 metres reduction is permitted (see IAN 111/09 Table 10-1 [Ref 8]). For roads other than Smart Motorways, where site constraints mean that none of the listed options on tolerances can be met, there is no formal departure process for the use of site-specific designs. The designer should retain evidence of the factors and assessments used to identify specific designs in the asset safety file.

2.10 Where even this cannot be achieved, then the ADS may be may be placed 1/3 mile and 2/3 mile in advance of the final ADS. When this is necessary it should be remembered that this gives drivers less time to carry out the weaving manoeuvre prior to the EDP. Consequently, the +10% tolerance should be applied in the placing of signs at 2/3 mile and 1/3 mile, where possible. A mixture of these distances, e.g. 1 mile followed by 1/3 mile should not be used.

2.11 On carriageways with 4 or more lanes, the first ADS should be located at 1 mile, owing to the additional time required to carry out the weaving manoeuvre prior to the EDP. At 70 mph, a motorist travels 1 mile in 51 seconds, but takes only 34 seconds to travel 2/3 mile. If it is necessary to use a 2/3 mile ADS on a carriageway with 4 lanes or more, then the provision of an additional ADS, for example at 11/2 miles, should be considered.

2.12 In some cases, motorway junctions are closely spaced, and there may be a need for overlap between advance direction signs for successive junctions. Section 5 gives examples of signing provision at closely spaced motorway junctions.

Countdown Marker Signs
2.13 Countdown marker signs (diagram 823, 824 & 825) shall be provided on the approach to taper diverge junctions, but shall not be provided at ‘Lane Drop’ junctions. Where provided, countdown marker signs are measured back from the EDP.
Traffic signs are generally the last bits of kit to go in. Unfortunately they are also usually down the list of priorities when people are fighting for the same space in the verge, along with the drainage, comms ducts, electric feeds for lighting and road restraint barrier. Therefore setting a sign at exactly 1 mile is subject to all sorts of outside forces, not least achieving the required level of visibility. Note also that the 1/2 mile and 1 mile signs for a junction will often be outside the red line boundary due to their distance from the actual geometric changes to a highway and will likely have been overlooked by highway designers who are more often concentrating on getting the alignment right, minimising earthworks, etc. than the placement of a few signs.

Having said that, siting the 300, 200, 100 yard countdowns should generally be easier and be at those exact distances.
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Re: How accurate are distance signs?

Post by ChrisH »

Debaser wrote: Mon Feb 01, 2021 13:06 Guidance on the location of signs on motorways and all-purpose trunk roads used to be in IAN144 (I don't know which of the new DMRB standards it will now be in).


Traffic signs are generally the last bits of kit to go in. Unfortunately they are also usually down the list of priorities when people are fighting for the same space in the verge, along with the drainage, comms ducts, electric feeds for lighting and road restraint barrier. Therefore setting a sign at exactly 1 mile is subject to all sorts of outside forces, not least achieving the required level of visibility. Note also that the 1/2 mile and 1 mile signs for a junction will often be outside the red line boundary due to their distance from the actual geometric changes to a highway and will likely have been overlooked by highway designers who are more often concentrating on getting the alignment right, minimising earthworks, etc. than the placement of a few signs.

Having said that, siting the 300, 200, 100 yard countdowns should generally be easier and be at those exact distances.
It's noticeable on the M4 smart motorway scheme that advance signs of the junctions have moved away from the standard 1m, 1/2m distances. J11 is signed at 2/3m and 1/3m, and J10 is signed at 1 1/4m and then 1/2m - presumably for the reasons you state. The new lane drops at these junctions have also meant the signs went in further back from the junction than previously.

On a different note, the most inaccurate distance sign I know of is driving anticlockwise around the M25 from the M11 to the M4: the distance to Heathrow seems to jump down and up and then down again.
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Re: How accurate are distance signs?

Post by Debaser »

ChrisH wrote: Mon Feb 01, 2021 13:19
Debaser wrote: Mon Feb 01, 2021 13:06 Guidance on the location of signs on motorways and all-purpose trunk roads used to be in IAN144 (I don't know which of the new DMRB standards it will now be in).


Traffic signs are generally the last bits of kit to go in. Unfortunately they are also usually down the list of priorities when people are fighting for the same space in the verge, along with the drainage, comms ducts, electric feeds for lighting and road restraint barrier. Therefore setting a sign at exactly 1 mile is subject to all sorts of outside forces, not least achieving the required level of visibility. Note also that the 1/2 mile and 1 mile signs for a junction will often be outside the red line boundary due to their distance from the actual geometric changes to a highway and will likely have been overlooked by highway designers who are more often concentrating on getting the alignment right, minimising earthworks, etc. than the placement of a few signs.

Having said that, siting the 300, 200, 100 yard countdowns should generally be easier and be at those exact distances.
It's noticeable on the M4 smart motorway scheme that advance signs of the junctions have moved away from the standard 1m, 1/2m distances. J11 is signed at 2/3m and 1/3m, and J10 is signed at 1 1/4m and then 1/2m - presumably for the reasons you state. The new lane drops at these junctions have also meant the signs went in further back from the junction than previously.

On a different note, the most inaccurate distance sign I know of is driving anticlockwise around the M25 from the M11 to the M4: the distance to Heathrow seems to jump down and up and then down again.
Smart motorway signage has the added complication that the fixed signs need to be certain distances from the VMS. And those comms designers do seem to love shifting their signs right to the last minute, so the (fixed) sign designer has to follow them, and good luck with that! Ultimately, if no land is available or would be unsafe (e.g. you're impinging on another junction if you use the standard distances) it comes down to risk assessing whether two signs at 'odd' mileages are better than a single one at the correct mileage - my opinion is they are (if only because I don't think the average driver can actually gauge distances when travelling at motorway speeds, therefore two signs gives additional warning), others opinions may vary.

Distances on route confirmatory signs are another matter. If (big if) the post office in the target town exists in its original location, it could be a thankless task to measure the distance to it using Streetview if it's 50 miles away and if you use a route planner, can you guarantee it's using the right roads? And of course, what if it's gone completely? I don't think there's any guidance on what to use as a datum point then.
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Re: How accurate are distance signs?

Post by ChrisH »

Debaser wrote: Mon Feb 01, 2021 13:35
ChrisH wrote: Mon Feb 01, 2021 13:19 On a different note, the most inaccurate distance sign I know of is driving anticlockwise around the M25 from the M11 to the M4: the distance to Heathrow seems to jump down and up and then down again.
Distances on route confirmatory signs are another matter. If (big if) the post office in the target town exists in its original location, it could be a thankless task to measure the distance to it using Streetview if it's 50 miles away and if you use a route planner, can you guarantee it's using the right roads? And of course, what if it's gone completely? I don't think there's any guidance on what to use as a datum point then.
On the M25 anticlockwise:

After the M11 junction, Heathrow is listed as 41 miles away. The only route that is 41 miles is M25, A1, A406, M4. The obvious route via M25 and M4 is 46 miles.

After the A10 junction, Heathrow and the M4 are both listed as 40 miles away. I make it 38 miles via the M25 and M4.

After the A1 junction, Heathrow is signed at 30 miles away. This seems correct.

After the M1 junction, Heathrow is signed as 22 miles away. This also seems correct.

Of course, as you say, the datum point and the routes to take are somewhat up for grabs, especially with such a huge place as Heathrow.

Now that I've dug all this I have reported it on the HE report-a-sign website :D
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Re: How accurate are distance signs?

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ChrisH wrote: Mon Feb 01, 2021 13:19 On a different note, the most inaccurate distance sign I know of is driving anticlockwise around the M25 from the M11 to the M4: the distance to Heathrow seems to jump down and up and then down again.
This sign on the length you refer to, which is all over the place!!!

It is impossible for Heathrow to be the same distance as the M4, given you have you use the M4 to get to Terminals (1/2) and 3, and J14 past the M4 to get to Terminals 4 and 5.

I was on it last week and measured from J25, M40 is 30 miles, M4 35 miles
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Re: How accurate are distance signs?

Post by Having a cuppa »

Stevie D wrote: Mon Feb 01, 2021 11:50
Having a cuppa wrote: Mon Feb 01, 2021 03:36
At least 40 mph will always be 40 mph no matter what.
Or within the +0/-10% tolerance allowed on the speedometer 🤔
What I meant was when Highways England erects a sign reading "1 mile", it will not always be one mile. I don't mean like a few yards off, I'm referring to a more considerable distance. At least when there is a 40 mph sign, it will always be referring to 40 miles per hour.
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Re: How accurate are distance signs?

Post by James1 »

Adopting the metric system would also solve this problem...
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Re: How accurate are distance signs?

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James1 wrote: Mon Feb 01, 2021 22:29 Adopting the metric system would also solve this problem...
:pig: :pig: :pig: :pig: :pig: :pig: :pig: :pig: :pig: :pig: :pig: :pig: :pig: :pig: :pig:

:D
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Re: How accurate are distance signs?

Post by Having a cuppa »

James1 wrote: Mon Feb 01, 2021 22:29 Adopting the metric system would also solve this problem...
Now I don't intend to spark a debate, but besides from the financial cost and there being no real benefits to adopting kilometres on road signage which outweigh the cost, me and my mates prefer our miles and pints, and I think that goes for most Brits as well. Not to mention a YouGov survey from a few years ago showed the overwhelming majority of those residing in the United Kingdom (90%) think in miles.
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Re: How accurate are distance signs?

Post by Chris Bertram »

Having a cuppa wrote: Tue Feb 02, 2021 01:11
James1 wrote: Mon Feb 01, 2021 22:29 Adopting the metric system would also solve this problem...
Now I don't intend to spark a debate, but besides from the financial cost and there being no real benefits to adopting kilometres on road signage which outweigh the cost, me and my mates prefer our miles and pints, and I think that goes for most Brits as well. Not to mention a YouGov survey from a few years ago showed the overwhelming majority of those residing in the United Kingdom (90%) think in miles.
Well, when you sign all distances in miles, that's kind of inevitable, wouldn't you think?
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Re: How accurate are distance signs?

Post by Debaser »

Having a cuppa wrote: Mon Feb 01, 2021 20:46 What I meant was when Highways England erects a sign reading "1 mile", it will not always be one mile. I don't mean like a few yards off, I'm referring to a more considerable distance.
This begs the question "How accurate do you think they should be"? As an ex-colleague from another branch of civil engineering once said, "We're building a road, not a Swiss watch".

Other than width and height restriction signs (which themselves are not exact, but incorporate a factor of safety in the rounding) does it matter if a 1 mile sign is 1609m from the datum point, 1588m or 1771m (both of which are 1m beyond the allowable tolerances). My concern would be that the distance between a 1 mile and the next 1/2 mile sign should be as close as possible to the distance between the 1/2 mile and final ADS, to enable drivers to better guesstimate the time they've got to make a manoeuvre, the same for a 2/3, 1/3, 0 set of signs.
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Re: How accurate are distance signs?

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Chris Bertram wrote: Tue Feb 02, 2021 07:59
Having a cuppa wrote: Tue Feb 02, 2021 01:11
James1 wrote: Mon Feb 01, 2021 22:29 Adopting the metric system would also solve this problem...
Now I don't intend to spark a debate, but besides from the financial cost and there being no real benefits to adopting kilometres on road signage which outweigh the cost, me and my mates prefer our miles and pints, and I think that goes for most Brits as well. Not to mention a YouGov survey from a few years ago showed the overwhelming majority of those residing in the United Kingdom (90%) think in miles.
Well, when you sign all distances in miles, that's kind of inevitable, wouldn't you think?
Exactly, things like this are always gonna be unpopular at first. The changeover happened here in Ireland not that long ago and we weren't bankrupted over it and now most people think in km instead of miles, the same would happen in the UK. Plus the UK is mostly metric anyway, why not finish the transition??
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Re: How accurate are distance signs?

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Conekicker wrote: Mon Feb 01, 2021 23:41
James1 wrote: Mon Feb 01, 2021 22:29 Adopting the metric system would also solve this problem...
:pig: :pig: :pig: :pig: :pig: :pig: :pig: :pig: :pig: :pig: :pig: :pig: :pig: :pig: :pig:

:D
you're probably right :lol:
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Re: How accurate are distance signs?

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James1 wrote: Tue Feb 02, 2021 16:32Exactly, things like this are always gonna be unpopular at first. The changeover happened here in Ireland not that long ago and we weren't bankrupted over it and now most people think in km instead of miles, the same would happen in the UK. Plus the UK is mostly metric anyway, why not finish the transition??
Sadly, I have a feeling that the UK government would probably find a way to gold-plate the process to make it *much* more expensive than it should be - there'd be an insistence on dual-signing for a transitional period of 5 or 10 years, there'd be a consultation period before that where every conceivable lobby group would be allowed to pitch for new speed limits sensible or otherwise, there would be the need for changes to any VSL technology and speed detection equipment, vehicles would need to switch their digital speedos over (older vehicles with dual-unit analogue speedos should be ok), there would be the most expensive PR campaign you would ever have seen as the change went in ... and still some people would claim to be unaware of the changes as they were booked for doing 90mph in a 90km/h area.

But I'd still be happy to see it happen.
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Re: How accurate are distance signs?

Post by Gareth »

Whilst I'd be for it too, we shouldn't underestimate the scale of such a task. We're a big country with a hell of a lot of road signs.
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Re: How accurate are distance signs?

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James1 wrote: Mon Feb 01, 2021 22:29 Adopting the metric system would also solve this problem...
Why would signing distances in metres mean that the people who erect signs put them at exactly the point advertised? :stir:
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Re: How accurate are distance signs?

Post by Truvelo »

Regarding the countdown marker signs approaching GSJ's the distance between the 100yd marker and the start of the slip road is definitely not 100yds. It's more like 200. At virtually every junction the distance between the 100yd sign and the slip road is twice that of the 300 to 200 and 200 to 100 signs :@
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