A50/A500 corridor

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Peter Freeman
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Re: A50/A500 corridor

Post by Peter Freeman »

Ruperts Trooper wrote: Wed Aug 17, 2022 08:35
... surely reducing the minimum radius will just increase the tipping risk - and/or - reduce the maximum safe speed.
A graduated radius allows speed to be reduced as the curve tightens. It's what's done on a normal road curve: a transition into the curve, rather than changing immediately from a straight road to a constant single radius curve and back again. Why would you not do the same within an interchange?
Last edited by Peter Freeman on Wed Aug 17, 2022 09:18, edited 1 time in total.
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Ruperts Trooper
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Re: A50/A500 corridor

Post by Ruperts Trooper »

Peter Freeman wrote: Wed Aug 17, 2022 08:59
Ruperts Trooper wrote: Wed Aug 17, 2022 08:35
... surely reducing the minimum radius will just increase the tipping risk - and/or - reduce the maximum safe speed.
A graduated radius allows speed to be reduced as the curve tightens. It's what's done on a normal road curve: a transition into the curve, rather than changing immediately from a straight road to a constant single radius curve and back again.
I understand the geometry - but the maximum safe speed is determined by the tightest radius of the curve (all other things being equal) so changing from constant radius to a variable radius would slow vehicles at the tightest point and mean that those with low power-to-weight would be slower at the merge point than they are now as their acceleration is low.
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Re: A50/A500 corridor

Post by Peter Freeman »

Ruperts Trooper wrote: Wed Aug 17, 2022 09:18
I understand the geometry - but the maximum safe speed is determined by the tightest radius of the curve (all other things being equal) so changing from constant radius to a variable radius would slow vehicles at the tightest point and mean that those with low power-to-weight would be slower at the merge point than they are now as their acceleration is low.
There are two separate issues here. One is the tipping risk, the other is the northbound merge.

1. Yes, the speed at the tightest point of my suggested shape would be lower than now. But the overall risk throughout the turn would be lower. You would not hurtle unexpectedly into a tight curve.

2. Yes, your speed emerging from the loop would be slower, exacerbating the merge problem, but that could be dealt with by the longer on-ramp.
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Re: A50/A500 corridor

Post by Ruperts Trooper »

Peter Freeman wrote: Wed Aug 17, 2022 09:27
Ruperts Trooper wrote: Wed Aug 17, 2022 09:18
I understand the geometry - but the maximum safe speed is determined by the tightest radius of the curve (all other things being equal) so changing from constant radius to a variable radius would slow vehicles at the tightest point and mean that those with low power-to-weight would be slower at the merge point than they are now as their acceleration is low.
There are two separate issues here. One is the tipping risk, the other is the northbound merge.

1. Yes, the speed at the tightest point of my suggested shape would be lower than now. But the overall risk throughout the turn would be lower. You would not hurtle unexpectedly into a tight curve.

2. Yes, your speed emerging from the loop would be slower, exacerbating the merge problem, but that could be dealt with by the longer on-ramp.
I'm not convinced about 1: - bigger, heavier vehicles need appropriate speed as they enter the curve in order not to tip over at the tightest point - by increasing the radius at the entry/exit points that would encourage vehicles to enter faster than presently despite the fact they will need to be slower than presently at the tightest point.

Others have said that the tipping risk isn't the main issue, the merge speed differential is the problem - so leave the curve as it is and just increase the merge distance.
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Re: A50/A500 corridor

Post by Peter Freeman »

Ruperts Trooper wrote: Wed Aug 17, 2022 09:37 I'm not convinced about 1: - bigger, heavier vehicles need appropriate speed as they enter the curve in order not to tip over at the tightest point - by increasing the radius at the entry/exit points that would encourage vehicles to enter faster than presently despite the fact they will need to be slower than presently at the tightest point.
OK, your view may be valid. Consider however -
A. My somewhat-elliptical shape suggestion produces a greater length of loop during which the same number of degrees is rotated. The average curve tightness is therefore lower, which in itself helps.
B. In similar vein, simply making the loop larger, but still with constant radius, would help. Jackal thinks there's not enough space. I don't know the exact situation on the ground - only what I see on google earth (though I have driven this interchange a number of times).
Others have said that the tipping risk isn't the main issue, the merge speed differential is the problem - so leave the curve as it is and just increase the merge distance.
Yes - this is the simplest option (plus, perhaps, emphasised warnings). I have commented before on how many trucks reportedly tip over on UK roundabouts. There seems to be a reluctance to slow down, noticeable also in how many Sabre posts concern speed limits. ABD members?
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Re: A50/A500 corridor

Post by Peter Freeman »

https://www.google.com.au/maps/@-37.827 ... 384!8i8192

Regarding tipping over, etc, I wonder which part of these multiple warnings are unclear -
https://www.google.com.au/maps/@52.9756 ... 384!8i8192

(though you could say that the placing of those temporary roadworks 60 signs was very unfortunate ... !)
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Re: A50/A500 corridor

Post by jackal »

Peter Freeman wrote: Wed Aug 17, 2022 08:14 ^ It looks to me that there is space, though not a lot. The northbound off-ramp could curve further left, with its current initial radius, and so come closer to the minor road to the south. This makes room for it, and thereby for the on-ramp also, to reach farther to the south and west. Both ramps, under my re-shape, would have, at their sharpest point, a slightly smaller radius than now, but that point would be approached and left on a larger radius (the loop would be somewhat elliptical).

Agreed, the gain is small, but at relatively low cost.

Which ramp actually causes the most trouble? Northbound I know, but on or off?

(reversing the trumpet takes you into major earthworks)
I agree you could slightly increase the radius (hence 'close to the maximum'). Maybe from 45m to 55m. But if they were to rebuild it, it would be extremely unlikely to be with such a tight radius. 75m radius seems to be the entry level for a freeflow loop nowadays.
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Re: A50/A500 corridor

Post by KeithW »

jackal wrote: Wed Aug 17, 2022 16:19 I agree you could slightly increase the radius (hence 'close to the maximum'). Maybe from 45m to 55m. But if they were to rebuild it, it would be extremely unlikely to be with such a tight radius. 75m radius seems to be the entry level for a freeflow loop nowadays.
We see the occasional truck on its side here.
https://www.google.co.uk/maps/@54.56256 ... !1e3?hl=en
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Re: A50/A500 corridor

Post by Peter Freeman »

KeithW wrote: Wed Aug 17, 2022 17:12
jackal wrote: Wed Aug 17, 2022 16:19 ... 75m radius seems to be the entry level for a freeflow loop nowadays.
We see the occasional truck on its side here.
https://www.google.co.uk/maps/@54.56256 ... !1e3?hl=en
That A19 one is 90m diameter / 45m radius, exactly the same as M6J15.

I've just measured the size of six similar loops, in the form of approx 270 degree trumpet or clover loops, on Melbourne's motorways. Only two are motorway-to-motorway. Four are on parclo service interchanges, with only one of these being encountered from the motorway. Results:

1 x 100m radius (M1/M2)
1 x 60m radius (M1/Wellington)
4 x 50m radius (M80/M2, M80/Melrose, M1/Sturt, M1/Koroit)

So most of ours are relatively tight too. The largest, as might be expected, is the most recently constructed (2001). The rest are relatively old (1970's and 1980's). All are urban.
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Re: A50/A500 corridor

Post by jackal »

A recent technical report on Simister Island refers at p. 18 to the design as "A small loop (radius of 100m)":

https://infrastructure.planninginspecto ... ERSION.pdf

IIRC two of the options for M25 J28 were 75m radius loops (the selected option is an irregular shape, perhaps with the same minimum radius).

Going back to a 00s scheme, the loop at M62/M57 is vast (160m?).

And as previously mentioned, the biggest loops in the country are of older vintage, on the M6 at the M42 and M69.
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Re: A50/A500 corridor

Post by Peter Freeman »

^ Very interesting. So a radius 100m is deemed 'small' ! The streetview image that I linked upthread (https://www.google.com.au/maps/@-37.827 ... 384!8i8192), suggesting a slow-down to 50km/hr, is for a loop of exactly that size.

Despite the great variations you've documented in loop size, and acknowledging that small can be too small, I simply don't believe that building loops that can be negotiated at motorway speed makes sense. Why should any motor vehicle journey, even if using only motorways or other limited access roads, be guaranteed, at great cost in $$ or land-take, immunity from the need to reduce speed or stop for an interchange? And yes, I do include the word 'stop'. Why not, if it contributes to value? A stop for, say, a minute or two, is not a great imposition during a long journey typical of motorways.

The older UK loops that you mention, which are larger still, perhaps reflect a previous even stronger belief in the inviolability of speed standards. A belief perhaps now pragmatically watered down??
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Re: A50/A500 corridor

Post by KeithW »

In the case of the A66/A19 GSJ the simple fact is that the max size was constrained by its location, we are lucky they didn't just use something like the Newport Roundabout.
https://www.google.co.uk/maps/@54.57080 ... !1e3?hl=en
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Re: A50/A500 corridor

Post by jackal »

Peter Freeman wrote: Thu Aug 18, 2022 11:29 ^ Very interesting. So a radius 100m is deemed 'small' ! The streetview image that I linked upthread (https://www.google.com.au/maps/@-37.827 ... 384!8i8192), suggesting a slow-down to 50km/hr, is for a loop of exactly that size.

Despite the great variations you've documented in loop size, and acknowledging that small can be too small, I simply don't believe that building loops that can be negotiated at motorway speed makes sense. Why should any motor vehicle journey, even if using only motorways or other limited access roads, be guaranteed, at great cost in $$ or land-take, immunity from the need to reduce speed or stop for an interchange? And yes, I do include the word 'stop'. Why not, if it contributes to value? A stop for, say, a minute or two, is not a great imposition during a long journey typical of motorways.

The older UK loops that you mention, which are larger still, perhaps reflect a previous even stronger belief in the inviolability of speed standards. A belief perhaps now pragmatically watered down??
Even a large loop has a low radius compared to a decently designed freeflow left turn or direct connector - for instance the Simister report I linked mentions that "Option C2-2 is proposed to reduce the radius of the M60 eastbound to M66 northbound interchange link from 360 m (as proposed in Option C2-1) to 255 m".

Hence all loops are designed so that traffic will have to slow down. The largest loop in the UK, the 250m radius M6/M42, culminates in a 35mph advisory, which is pretty baffling, but is presumably not really an attempt to get traffic to go at 35mph but rather there to scare boy racers into going down to 60 or so. The next largest, the 170m M57/M62, has a 50 advisory.

When we're talking about more normal-sized loops the question is really whether 30mph or 40mph is the target. For an interchange between two grade-separated roads 40mph should be the minimum for a new build without severe space constraints, which means somewhere around 75m radius. The M8/A725 loop is a good recent example. A 30mph/50m radius loop introduces safety risks and increased journey times disproportionate to the minor saving in land take compared to 75m.

PS - I made a loop thread here: viewtopic.php?t=43792
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Re: A50/A500 corridor

Post by Ruperts Trooper »

jackal wrote: Fri Aug 19, 2022 15:06
Peter Freeman wrote: Thu Aug 18, 2022 11:29 ^ Very interesting. So a radius 100m is deemed 'small' ! The streetview image that I linked upthread (https://www.google.com.au/maps/@-37.827 ... 384!8i8192), suggesting a slow-down to 50km/hr, is for a loop of exactly that size.

Despite the great variations you've documented in loop size, and acknowledging that small can be too small, I simply don't believe that building loops that can be negotiated at motorway speed makes sense. Why should any motor vehicle journey, even if using only motorways or other limited access roads, be guaranteed, at great cost in $$ or land-take, immunity from the need to reduce speed or stop for an interchange? And yes, I do include the word 'stop'. Why not, if it contributes to value? A stop for, say, a minute or two, is not a great imposition during a long journey typical of motorways.

The older UK loops that you mention, which are larger still, perhaps reflect a previous even stronger belief in the inviolability of speed standards. A belief perhaps now pragmatically watered down??
Even a large loop has a low radius compared to a decently designed freeflow left turn or direct connector - for instance the Simister report I linked mentions that "Option C2-2 is proposed to reduce the radius of the M60 eastbound to M66 northbound interchange link from 360 m (as proposed in Option C2-1) to 255 m".

Hence all loops are designed so that traffic will have to slow down. The largest loop in the UK, the 250m radius M6/M42, culminates in a 35mph advisory, which is pretty baffling, but is presumably not really an attempt to get traffic to go at 35mph but rather there to scare boy racers into going down to 60 or so. The next largest, the 170m M57/M62, has a 50 advisory.

When we're talking about more normal-sized loops the question is really whether 30mph or 40mph is the target. For an interchange between two grade-separated roads 40mph should be the minimum for a new build without severe space constraints, which means somewhere around 75m radius. The M8/A725 loop is a good recent example. A 30mph/50m radius loop introduces safety risks and increased journey times disproportionate to the minor saving in land take compared to 75m.

PS - I made a loop thread here: viewtopic.php?t=43792
Many drivers take the nature of their vehicle and the conditions into account when seeing advisory speeds on bends with only the biggest, heaviest actually slowing to the recommended speed - many modern cars can get round at double the posted advisory in good conditions but a top-heavy HGV could be in real trouble anywhere above it.
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Re: A50/A500 corridor

Post by Peter Freeman »

Ruperts Trooper wrote: Fri Aug 19, 2022 15:33 Many drivers take the nature of their vehicle and the conditions into account when seeing advisory speeds on bends with only the biggest, heaviest actually slowing to the recommended speed - many modern cars can get round at double the posted advisory in good conditions but a top-heavy HGV could be in real trouble anywhere above it.
I'm sure most drivers do that perfectly sensible calculation. But if we are going to sign a speed recommendation (and for most freeflow loops, we should), it must address close-to-the-worst case. The traditional, even subconscious, calculation to customise your figure will thus remain valid.
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Re: A50/A500 corridor

Post by JF2309 »

The only way a radical change at Sideway will happen is if the Council Incinerator is moved to a new site when the current incinerator is replaced at the end of the decade. This isn’t taking into account how close the Trentham Lakes junction is to the roundabout. But something has to happen and a bullet needs to be bitten.

J15 is desperately in need of being smashed to atoms and replaced, there is a whacking great hill in the way and a Warren of other roads that plug into and around J15, how they go about this will be more about how much will there is to do it properly over how badly it needs doing.

J16 by virtue of it not being a mess isn’t in as dire need but I can see a day coming where links like those at Lofthouse from M6S>A500S/A500N>M6N will be needed.

The Sudbury roundabout on the A50 is a glaring omission from this.

But the big take from all of this is that the M64 would have saved a lot of money, time and effort 40 years later.
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Re: A50/A500 corridor

Post by KeithW »

Peter Freeman wrote: Sat Aug 20, 2022 03:15 I'm sure most drivers do that perfectly sensible calculation. But if we are going to sign a speed recommendation (and for most freeflow loops, we should), it must address close-to-the-worst case. The traditional, even subconscious, calculation to customise your figure will thus remain valid.
We do but shock horror some drivers don't obey speed limit signs or even seem to look at what is in front of them !
https://www.google.co.uk/maps/@54.56290 ... 8192?hl=en

Rule number one on the road - Drive according to the conditions and the characteristics of your vehicle, the speed limit is the maximum allowed not a target or recommendation.
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Re: A50/A500 corridor

Post by Big L »

KeithW wrote: Sat Aug 20, 2022 12:00
Peter Freeman wrote: Sat Aug 20, 2022 03:15 I'm sure most drivers do that perfectly sensible calculation. But if we are going to sign a speed recommendation (and for most freeflow loops, we should), it must address close-to-the-worst case. The traditional, even subconscious, calculation to customise your figure will thus remain valid.
We do but shock horror some drivers don't obey speed limit signs or even seem to look at what is in front of them !
https://www.google.co.uk/maps/@54.56290 ... 8192?hl=en

Rule number one on the road - Drive according to the conditions and the characteristics of your vehicle, the speed limit is the maximum allowed not a target or recommendation.
But if everybody obeyed rule number one there wouldn’t be drivers rolling over because they took bends too quickly.
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Re: A50/A500 corridor

Post by Bryn666 »

I remember getting lots of grief for signposting the A556 to M56 loop as an advisory 30. Apparently the tried and tested method of "what speed can you do before G forces make you soil yourself and deduct 10" is not in vogue with the spreadsheet wonks.
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Re: A50/A500 corridor

Post by KeithW »

Big L wrote: Sat Aug 20, 2022 12:10 But if everybody obeyed rule number one there wouldn’t be drivers rolling over because they took bends too quickly.
My point precisely, anyone who rolls over like that should go on a remedial driving course.
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