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Peter Freeman wrote:As a more general comment, I have often thought that adding junctions would solve some problems on UK motorways. For a densely-populated island the average junction spacing has always seemed rather wide to me. To simply convert a nearby crossing road into a junction by adding ramps (no new structure) would often be possible, and would offer cost-effective relief to a struggling nearby intersection that is difficult to upgrade. This M20 proposal is almost my wish, except that the structures are new.
I agree with a lot of your thinking, but I'm not sure this idea works so well. One of our major problems is that our motorway network is already operating at or over capacity, particularly near cities. Adding more junctions encourages mor traffic onto motorways and encourages traffic to make mor short-hop, local journeys. I think adding more junctions would be likely to make the situation considerably worse on what are supposed to be through routes.
I'd rather see provision of better local and regional roads in the areas around cities so commuter and local traffic relies less on the national motorway network.
Oh, just replying to my own post after re-checking the scheme layout! The two Hythe Road (A20) arms on the north side of the new two-bridge roundabout makes substituting a diamond awkward, and a diverging diamond even more so. Perhaps a dumbbell instead? (I hear some groans ...).
Also, with Hythe Road linking J10 to J10a, perhaps entirely removing the eastern ramps of J10 IS a good idea after all.
Peter Freeman wrote:Also, with Hythe Road linking J10 to J10a, perhaps entirely removing the eastern ramps of J10 IS a good idea after all.
Actually I think you were right first time. Hythe Rd has a roundabout for accessing the Tesco Extra, a hospice entrance and numerous private accesses on it, so it is not really a good option for getting between the two motorway junctions - signs will surely direct via the new roundabout and DC to the south.
As you originally said, they should have merged the east-facing slips at J10 and west-facing slips at J10a, with a diverge/merge to/from the M20. In fact you can see from the plan that the two sets of slips overlap, so they will end up building almost all of the necessary infrastructure, but then close half of it (the J10 east-facing slips). This is, as Alan Partridge might put it, tantamount to vandalism.
Peter Freeman wrote:As a more general comment, I have often thought that adding junctions would solve some problems on UK motorways. For a densely-populated island the average junction spacing has always seemed rather wide to me. To simply convert a nearby crossing road into a junction by adding ramps (no new structure) would often be possible, and would offer cost-effective relief to a struggling nearby intersection that is difficult to upgrade. This M20 proposal is almost my wish, except that the structures are new.
When I was walking along the B4120 to improve its wiki article, I thought "If Barnt Green had an M42 junction, this road would be overloaded with traffic trying to get into Birmingham". Perhaps I'm wrong – the traffic might well choose to use the A441 instead – but the problem with adding junctions is that it encourages motorway traffic to load the roads around the junctions. This might be even more of a problem than local traffic overloading the motorway.
I'm reminded of Braess' paradox, which is something that everyone on SABRE should be aware of. (Summary: building a new road can actually slow down journeys on average.)
Peter Freeman wrote:As a more general comment, I have often thought that adding junctions would solve some problems on UK motorways. For a densely-populated island the average junction spacing has always seemed rather wide to me. To simply convert a nearby crossing road into a junction by adding ramps (no new structure) would often be possible, and would offer cost-effective relief to a struggling nearby intersection that is difficult to upgrade. This M20 proposal is almost my wish, except that the structures are new.
When I was walking along the B4120 to improve its wiki article, I thought "If Barnt Green had an M42 junction, this road would be overloaded with traffic trying to get into Birmingham". Perhaps I'm wrong – the traffic might well choose to use the A441 instead – but the problem with adding junctions is that it encourages motorway traffic to load the roads around the junctions. This might be even more of a problem than local traffic overloading the motorway.
I'm reminded of Braess' paradox, which is something that everyone on SABRE should be aware of. (Summary: building a new road can actually slow down journeys on average.)
Most local motorway junctions (i.e. service interchanges) in the UK are full access. I think it would be better to build more junctions but with limited access. So instead of having all traffic for the motorway going along local road A, you split it between road A for west-facing slips and road B for east-facing slips, for instance.
This would not create a Braess paradox, as that occurs from excessive traffic cramming into one road (e.g. a 'shortcut'). The opposite occurs here - you are spreading traffic over more routes.
A more serious problem is that, clearly, you are going to increase traffic on the second local road ('road B'). Where the full access junction has already been provided, and people are used to road A being busy and road B being quiet, it might be hard to justify dividing the load between the two roads, especially to people who live on or near road B. So it is probably easier to do as you build the motorway rather than as a congestion relief scheme at a later point.
Hi. I am new to this forum, and must declare an interest - I live a mile away from Junction 10, and use it a lot.
Here's the thing - when in 2008 this proposal came up for consultation, we locals objected to the closing of the two east-facing sliproads. We saw that it meant that loads of traffic which currently uses the sliproads would have to go along the old A20 with its Tesco pinch point.
Nothing's changed, and at the public display I was told that there were no plans to upgrade the A20.
So we're looking at local traffic plus HGV's negotiating a tiny roundabout - talk about unfit for purpose!
What they seem to be saying is that they MUST remove the sliproads because the old and new junctions will only be 700m apart. We have to live with the consequences: it's not their fault.
As a layman, I don't understand this. Are they right?
stephenfryer wrote:Hi. I am new to this forum, and must declare an interest - I live a mile away from Junction 10, and use it a lot.
Here's the thing - when in 2008 this proposal came up for consultation, we locals objected to the closing of the two east-facing sliproads. We saw that it meant that loads of traffic which currently uses the sliproads would have to go along the old A20 with its Tesco pinch point.
Nothing's changed, and at the public display I was told that there were no plans to upgrade the A20.
So we're looking at local traffic plus HGV's negotiating a tiny roundabout - talk about unfit for purpose!
What they seem to be saying is that they MUST remove the sliproads because the old and new junctions will only be 700m apart. We have to live with the consequences: it's not their fault.
As a layman, I don't understand this. Are they right?
They are right that the east-facing slips cannot remain in their current form if they build J10a. But they're not right that that means they have to go altogether. They could extend the slips so they join the two roundabouts together, a bit like this. That way there'd be no need to rat run past Tesco to get between the roundabouts. The only reason I can think of why they're not doing this is as a (very small) cost saving.
Do you think that something has to be done about the east-facing slips because it's common sense that two slips 700m apart is a dangerous idea? Or is there a legal calculation of distance: what longer length would keep the slips?
To join the two roundabouts together might entail shaving a bit off the Tesco car park, and they would object I'm sure. Is there a minimum width for slips?
imo, the best solution would be to build J10a as far as is necessary east to retain the slips, and move the roundabout on the A2070 to the Barrey Road exit. This would solve both the rat-run and Barrey Road problems. Can you see anything wrong with that?
Do you think that something has to be done about the east-facing slips because it's common sense that two slips 700m apart is a dangerous idea? Or is there a legal calculation of distance: what longer length would keep the slips?
To join the two roundabouts together might entail shaving a bit off the Tesco car park, and they would object I'm sure. Is there a minimum width for slips?
imo, the best solution would be to build J10a as far as is necessary east to retain the slips, and move the roundabout on the A2070 to the Barrey Road exit. This would solve both the rat-run and Barrey Road problems. Can you see anything wrong with that?
Merging and leaving traffic travelling at 70mph needs quite distance to sort itself out and as such it has long been accepted that best practice has at least a mile between slip roads joining and the next set leaving the motorway. If they get too close then the speed limit has to be reduced to slow everyone down and give them time to shuffle themselves around in the shorter than normal distance provided - and that is not a sensible solution on a rural (as opposed to urban) motorway.
No, the only safe and viable solution is to close the direct east facing slips slips to / from the M20 at the existing junction (unless you start adding in braided sliproads as found on the M61 close to Bolton) and no amount of campaigning will change that. However that does not mean an arrangement like those already posted on here by others (the M25 / A282 junction) cannot be employed to alleviate problems posed by the current plans.
Do you think that something has to be done about the east-facing slips because it's common sense that two slips 700m apart is a dangerous idea? Or is there a legal calculation of distance: what longer length would keep the slips?
There are actually specific numbers. Merge to diverge (i.e. a slip road joining, to a slip road leaving) has the longest distance; on typical motorways, it's 2 kilometers (you can sometimes go lower on an "urban motorway" because they have different design standards and typically lower speed limits; there's a formula involved). The other combinations (e.g. the minimum distance between two slip roads merging) have a minimum distance of 422.36 metres (produced by a formula based on the speed limit of 70 mph).
Additionally, for weaving lengths close to the minimum (e.g. the 2 kilometers in this case), the design guidance sometimes requires making the road wider to compensate (so that vehicles that are going straight on can move to the right so that they don't get in the way of vehicles involved in the weaving). Presumably this would involve designing the weaving section as a lane gain followed by a lane drop, which would tend to naturally make traffic that was going straight on move further to the right.
Last edited by ais523 on Tue Feb 09, 2016 23:45, edited 1 time in total.
ais523 wrote:I'm reminded of Braess' paradox, which is something that everyone on SABRE should be aware of. (Summary: building a new road can actually slow down journeys on average.)
I've never heard of this paradox. I'm also rather skeptical of it, though I'm sure that in certain circumstances such slowdowns might result.
ais523 wrote:I'm reminded of Braess' paradox, which is something that everyone on SABRE should be aware of. (Summary: building a new road can actually slow down journeys on average.)
I've never heard of this paradox. I'm also rather skeptical of it, though I'm sure that in certain circumstances such slowdowns might result.
Please explain - and who was Braess?
There's a link in my post, but as a summary: suppose you have a fast road that has a tendency to get overloaded, but even when it has a traffic jam it's still faster than the alternatives. If you make it easier to access by building new roads that connect to it, it'll get even more heavily loaded and even slower with new routes diverting onto it (because it's faster), and it'll still be the best route for the journeys that were using it already, but those journeys will now be slower due to the added congestion.
If you have two such roads, and link them together, it's possible that the best route in the resulting road network is to use both (thus slowing the roads down heavily), whereas if they weren't linked, the "second-best" route of using one of the fast roads and then side roads for the rest of the journey would actually end up faster overall because there'd be less traffic on the fast roads in the first place.
Braess appears to be famous for the paradox and not much else.
Do you think that something has to be done about the east-facing slips because it's common sense that two slips 700m apart is a dangerous idea? Or is there a legal calculation of distance: what longer length would keep the slips?
They wouldn't be 700m apart, if you look at the plans the current east-facing and proposed west-facing slips actually overlap. This is part of the reason why joining the two roundabouts together is such a natural solution - the amount of additional works required would be small.
To join the two roundabouts together might entail shaving a bit off the Tesco car park, and they would object I'm sure. Is there a minimum width for slips?
If you look here, you'll see the current slips extend past Tesco, so there would be no need to impinge on their business in order to extend the slips to the new roundabout.
imo, the best solution would be to build J10a as far as is necessary east to retain the slips, and move the roundabout on the A2070 to the Barrey Road exit. This would solve both the rat-run and Barrey Road problems. Can you see anything wrong with that?
As ais523 says, this would mean J10a would have to be built 2km+ away to the east, making it almost useless in terms of relieving J10.
I am grateful to you all. It seems my best option is to focus on persuading them to retain the sliproads and extend them to link the two Junctions, thus avoiding the Tesco rat-run. I'll let you know how it goes.
Another question, please, this time about the Barrey Road junction.
We locals have campaigned for years for a roundabout or traffic signals at its exit on to the A2070. At the moment, all traffic coming out from the trading estate must turn left: half of it wants to go south but has to go north and round junction 10 and come back. Also, the A2070 only has a 70mph speed limit and exiting from Barrey Road is terrifying - there have been many accidents but no fatalities - yet. Under the new proposals, where the new road from junction 10a joins the A2070 is to be a roundabout. We are told that this solves the problem, but we still think it unfair to make us head north to go south (albeit a lesser distance) and we remain worried about speeding traffic on the A2070.
Are we being unreasonable? Can you see any reason why they could not move the new roundabout to be at the Barrey Road exit? And can you see anything in the plans which will reduce the traffic speed?
stephenfryer wrote:We locals have campaigned for years for a roundabout or traffic signals at its exit on to the A2070. At the moment, all traffic coming out from the trading estate must turn left: half of it wants to go south but has to go north and round junction 10 and come back. Also, the A2070 only has a 70mph speed limit and exiting from Barrey Road is terrifying - there have been many accidents but no fatalities - yet. Under the new proposals, where the new road from junction 10a joins the A2070 is to be a roundabout. We are told that this solves the problem, but we still think it unfair to make us head north to go south (albeit a lesser distance) and we remain worried about speeding traffic on the A2070.
Are we being unreasonable? Can you see any reason why they could not move the new roundabout to be at the Barrey Road exit? And can you see anything in the plans which will reduce the traffic speed?
I would think the junction 10A plans will solve this problem entirely.
The new roundabout will be located immediately north of Barrey Road. This means that your distance to travel northbound before U-turning to go south is massively reduced, and the need to circumnavigate an enormous signalised roundabout is removed.
The new roundabout will also reduce the speed of traffic. It looks like it will be located about 200m north of Barrey Road, a point at which traffic will be slowing down on approach to the junction. It will no longer be safe or reasonable to travel at 70mph northbound at Barrey Road, so you'll be emerging into a stream of slower-moving vehicles.
Thankyou. On Barrey Road, we may be offered an acceleration lane which will help with the problem of emerging into traffic.
Yesterday at our local Community Forum, Salvatore Zappala, Highways England Project Manager for J10a, presented the plans. He sees the future of J10 as being for local traffic; of J10a as for national traffic. He will use signage to discourage (but not prevent) national traffic from using the stretch of the old A2070 between J10 and the new roundabout just north of Barrey Road. Signs will direct M20 traffic wishing to travel south to leave by the J10a slips, and the J10 slips have to go. No dumbbell, sorry...
We thought that traffic would continue to take the shortest route along the old A2070, and that sat navs would direct them thus. So, we were being forced into the narrow, two-way, congested Tesco rat-run. We could see no advantages for local traffic; some for national traffic.
The consultation period is almost over. We have made our case, and incorporated the dumbbell idea. We got that from you guys, so thanks. When we know more, I'll let you know.
One point: HE rely on traffic modelling figures to show that the increase in traffic on the A20 will be outweighed by fewer cars which will now leave the M20 at J10a. We think this is counter-intuitive but have no figures of our own.
So how does one get to see traffic modelling evidence, and how does one test it?
