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Bryn666 wrote: ↑Mon Apr 15, 2019 08:46The rest of the world is actually moving towards compact roundabouts for small junctions and high capacity signal arrangements like continuous flow for at grade and diverging diamonds for major grade separated junctions.
As usual Britain thinks it is better and stubbornly sticks to designs that were obsolescent twenty years ago.
Compact roundabouts for small junctions, I completely agree with. That's not appropriate for the A1237 though – the problem is that it had compact(ish) roundabouts and they didn't cope.
Continuous flow would certainly be worth looking at, but they do take up significantly more space than enlarged roundabouts, and there isn't a massive amount of space around some of the junctions on A1237 – plus several of the junctions are complicated by a fifth arm at the roundabout or something else that would make CF more difficult.
What is the capacity difference of a roundabout with 3-lane approach and circulation and 2-lane exit, compared to a signalised junction with similar land-take? Genuine question, I would be interested to know the maximum peak-flow throughput, typical peak-time delays and typical off-peak delays for different models.
The US FHWA says up to 45,000 AADT for a two-lane roundabout, that is entry from all arms. That would be about the level on the York northern ring road (25-30k on the ring road, the rest from side roads). They don't give generic figures for three lane (above two lane requires 'detailed capacity analysis') but should be adequate in this case. We have hundreds of roundabouts that operate fine at those levels. You are guaranteeing offpeak delays with any signalized junction so they should be used only as a last resort.
jackal wrote: ↑Mon Apr 15, 2019 11:07
The US FHWA says up to 45,000 AADT for a two-lane roundabout, that is entry from all arms. That would be about the level on the York northern ring road (25-30k on the ring road, the rest from side roads). They don't give generic figures for three lane (above two lane requires 'detailed capacity analysis') but should be adequate in this case. We have hundreds of roundabouts that operate fine at those levels. You are guaranteeing offpeak delays with any signalized junction so they should be used only as a last resort.
Most of the old A1 roundabouts were operating in the 40k AADT range and I can tell you they produced long delays very frequently as for US experience it should be noted that they replaced them with GSJ's in the Cape Cod area as they were also causing long delays. For the most part US roundabouts are heavily signalised. https://www.google.co.uk/maps/@40.76774 ... 8192?hl=en
Those that are not are on roads carrying local traffic such as the Vfw Circular in Boston which is really a Roundabout Interchange with the main route being grade separated. https://www.google.com/maps/@42.5638271 ... 6656?hl=en
In the case of York they have improved some roundabouts such as Rawcliffe by making them larger and adding left turn filter lanes. Traffic flows there are in the order of 30k on the ring road and 10k on the A19. This has helped but is I suspect a temporary solution at best. Make the ring road D2 and I guarantee that a GSJ will be needed.
The basic problem with York is that what was originally the southern bypass intended to take the A64 traffic out of the York local traffic flows has become part of the ring road so you once more have a mix of through traffic and local drivers. This is what makes Hopgrove such a mess. You have 10k on the A1036 to/from York centre, 35k on the A64 westbound , 25 k on the A64 Eastbound and 25k on the A1237 all trying to weave through 2 roundabouts only 100m apart.
Hopgrove had to be signalised because at certain times the A64 flow was so dominant that getting onto it from the A1237 was next to impossible causing long tailbacks that gridlocked the ring road. This is a pattern which has been repeated with many ring roads around the country. It is also extremely unfriendly for non motorised traffic such as cyclists.
KeithW wrote: ↑Mon Apr 15, 2019 13:03In the case of York they have improved some roundabouts such as Rawcliffe by making them larger and adding left turn filter lanes.
Kind of, kind of not.
They are not filter lanes, because traffic still has to give way to other traffic coming off the roundabout. It's something York Council has been doing for a few years now, on 3-lane approaches, the left-hand left-turn lane is separated by hatching and a traffic island from lanes 2 and 3, even though it still has to give way. I don't know whether this is to reduce people erroneously going straight on from the left lane, or whether it makes it pushes turning traffic far enough away from the centre of the roundabout that it can turn left while other traffic is making the opposite right-turn (ie coming from the right and leaving straight ahead), but either way it's a bit confusing when you first encounter it.
KeithW wrote: ↑Mon Apr 15, 2019 13:03In the case of York they have improved some roundabouts such as Rawcliffe by making them larger and adding left turn filter lanes.
Kind of, kind of not.
They are not filter lanes, because traffic still has to give way to other traffic coming off the roundabout. It's something York Council has been doing for a few years now, on 3-lane approaches, the left-hand left-turn lane is separated by hatching and a traffic island from lanes 2 and 3, even though it still has to give way. I don't know whether this is to reduce people erroneously going straight on from the left lane, or whether it makes it pushes turning traffic far enough away from the centre of the roundabout that it can turn left while other traffic is making the opposite right-turn (ie coming from the right and leaving straight ahead), but either way it's a bit confusing when you first encounter it.
Yes, I mentioned these semi-filters above. As you say, it should at least remove some conflicts between vehicles that are heading for different exits.
It seems to be an accident of geometry when they put a proper filter in, i.e. their preference is for a semi-filter but when it doesn't fit they have to settle for a higher capacity option!
Topologically, it's no different from the others, traffic turning left still has to give way, it's just a slightly bigger island separating it from the other flows but that's all.
Topologically, it's no different from the others, traffic turning left still has to give way, it's just a slightly bigger island separating it from the other flows but that's all.
The difference is that you're not giving way to traffic on the roundabout, you're giving way to traffic on the road proper. That's what makes it an actual filter.
An update. Dualling of the northern part of the A1237, from A19 Shipton Road to A1036 Little Hopgrove, is anticipated to start in mid-2023, with construction lasting 2 years. The improvement includes enlargement of roundabouts.
jackal wrote: ↑Thu Nov 11, 2021 15:43
An update. Dualling of the northern part of the A1237, from A19 Shipton Road to A1036 Little Hopgrove, is anticipated to start in mid-2023, with construction lasting 2 years. The improvement includes enlargement of roundabouts.
Typical... the half I use the least, as opposed to the A19 to A64 (Askham)
jackal wrote: ↑Thu Nov 11, 2021 15:43An update. Dualling of the northern part of the A1237, from A19 Shipton Road to A1036 Little Hopgrove, is anticipated to start in mid-2023, with construction lasting 2 years. The improvement includes enlargement of roundabouts.
No complaint, but I'm trying to understand why it's being part-funded by the West Yorkshire Combined Authority. Presumably WYCA includes York or parts of North Yorkshire, but if so it's very badly named! The WYCA website seems to be missing the key information about who and where it covers.
jackal wrote: ↑Thu Nov 11, 2021 15:43An update. Dualling of the northern part of the A1237, from A19 Shipton Road to A1036 Little Hopgrove, is anticipated to start in mid-2023, with construction lasting 2 years. The improvement includes enlargement of roundabouts.
No complaint, but I'm trying to understand why it's being part-funded by the West Yorkshire Combined Authority. Presumably WYCA includes York or parts of North Yorkshire, but if so it's very badly named! The WYCA website seems to be missing the key information about who and where it covers.
jackal wrote: ↑Thu Nov 11, 2021 15:43An update. Dualling of the northern part of the A1237, from A19 Shipton Road to A1036 Little Hopgrove, is anticipated to start in mid-2023, with construction lasting 2 years. The improvement includes enlargement of roundabouts.
No complaint, but I'm trying to understand why it's being part-funded by the West Yorkshire Combined Authority. Presumably WYCA includes York or parts of North Yorkshire, but if so it's very badly named! The WYCA website seems to be missing the key information about who and where it covers.
Given how much of the traffic heading for York, Little Hopgrove and the Yorkshire Coast originates in and around Bradford, Leeds, Wakefield and elsewhere in West Yorkshire I can see where they might have an interest. Traffic from Teesside tends to use the A19/A171 while trippers from Hull will probably use the A164 and A165. York was the central point at which West Riding North Riding and East Riding converged and historically was the county town of Yorkshire and is where most of the major roads in the county converge. The reality is York is a unitary authority and cannot realistically carry the burden of maintaining and upgrading all those roads that hit the A1237.
I believe the DfT also provided a substantial chunk of money for the scheme.
Last edited by KeithW on Thu Nov 11, 2021 18:06, edited 1 time in total.
jackal wrote: ↑Thu Nov 11, 2021 15:43An update. Dualling of the northern part of the A1237, from A19 Shipton Road to A1036 Little Hopgrove, is anticipated to start in mid-2023, with construction lasting 2 years. The improvement includes enlargement of roundabouts.
No complaint, but I'm trying to understand why it's being part-funded by the West Yorkshire Combined Authority. Presumably WYCA includes York or parts of North Yorkshire, but if so it's very badly named! The WYCA website seems to be missing the key information about who and where it covers.
Maybe it should be the West Metropolitan Yorkshire Combined Authority - the YMCA. Yo man!
I hope those roundabouts aren’t built with those geometries - a sure invitation for entering vehicles to drift across into the wrong lane into the path of vehicles alongside them.
jackal wrote: ↑Thu Nov 11, 2021 15:43An update. Dualling of the northern part of the A1237, from A19 Shipton Road to A1036 Little Hopgrove, is anticipated to start in mid-2023, with construction lasting 2 years. The improvement includes enlargement of roundabouts.
No complaint, but I'm trying to understand why it's being part-funded by the West Yorkshire Combined Authority. Presumably WYCA includes York or parts of North Yorkshire, but if so it's very badly named! The WYCA website seems to be missing the key information about who and where it covers.
I seem to think the West Yorkshire combined authority covers the areas that are in the Leeds city region, that includes Craven, Harrogate, Selby districts in North Yorkshire and City of York too.
Little Hopgrove should be a signalised junction, not a tiny signalised roundabout, like it is currently. There is more than enough space within the existing area for protected right turns and a give way for left, heading north off of Hopgrove
KeithW wrote: ↑Mon Apr 15, 2019 13:03
... as for US experience it should be noted that they replaced them with GSJ's in the Cape Cod area as they were also causing long delays. For the most part US roundabouts are heavily signalised. https://www.google.co.uk/maps/@40.76774 ... 8192?hl=en
Those that are not are on roads carrying local traffic such as the Vfw Circular in Boston which is really a Roundabout Interchange with the main route being grade separated. https://www.google.com/maps/@42.5638271 ... 6656?hl=en
I'm not arguing (here) with the main theme of your post, but those two USA roundabouts are a strange choice to use as illustrations.
Columbus Circle in NYC certainly is heavily signalised. However, it's about as a-typical as you could get. It's a beloved historical anomaly, but, if not already there, would not enter the mind as a grid intersection for that city.
The one in Massachusetts (and a similar intersection just down the road) is also totally a-typical. It's a copy of the standard UK type, and most inadvisably so, especially at that location. Perhaps some remnant 'New England' nostalgia for the old country crept into the planners' decision?
The typical USA 'modern roundabout' is small and un-signalised.
KeithW wrote: ↑Mon Apr 15, 2019 13:03
... as for US experience it should be noted that they replaced them with GSJ's in the Cape Cod area as they were also causing long delays. For the most part US roundabouts are heavily signalised. https://www.google.co.uk/maps/@40.76774 ... 8192?hl=en
Those that are not are on roads carrying local traffic such as the Vfw Circular in Boston which is really a Roundabout Interchange with the main route being grade separated. https://www.google.com/maps/@42.5638271 ... 6656?hl=en
I'm not arguing (here) with the main theme of your post, but those two USA roundabouts are a strange choice to use as illustrations.
Columbus Circle in NYC certainly is heavily signalised. However, it's about as a-typical as you could get. It's a beloved historical anomaly, but, if not already there, would not enter the mind as a grid intersection for that city.
The one in Massachusetts (and a similar intersection just down the road) is also totally a-typical. It's a copy of the standard UK type, and most inadvisably so, especially at that location. Perhaps some remnant 'New England' nostalgia for the old country crept into the planners' decision?
The typical USA 'modern roundabout' is small and un-signalised.
The Mass examples are actually rotaries, with the old entering traffic has priority design hence the long slip roads and fast corners. The yield on entry signs were added later as modern roundabouts started to arrive in the USA.
Bryn Terminally cynical, unimpressed, and nearly Middle Age already. She said life was like a motorway; dull, grey, and long.
Peter Freeman wrote: ↑Fri Nov 12, 2021 01:02
I'm not arguing (here) with the main theme of your post, but those two USA roundabouts are a strange choice to use as illustrations.
Columbus Circle in NYC certainly is heavily signalised. However, it's about as a-typical as you could get. It's a beloved historical anomaly, but, if not already there, would not enter the mind as a grid intersection for that city.
The one in Massachusetts (and a similar intersection just down the road) is also totally a-typical. It's a copy of the standard UK type, and most inadvisably so, especially at that location. Perhaps some remnant 'New England' nostalgia for the old country crept into the planners' decision?
The typical USA 'modern roundabout' is small and un-signalised.
There were originally several rotaries in that part of New England that have been gradually replaced over the years. One of the most notorious was at Sagamore Bridge that was replaced by a GSJ in 2006.
Rotaries have seen something of a resurgence in Massachusetts where they actually building new examples. Here is one in Braintree MA. That said its one of the few parts of the USA where population density is more like that in the UK. I lived in Ohio were the population density is about 20% of that in England and its mostly flat.
