The study of British and Irish roads - their construction, numbering, history, mapping, past and future official roads proposals and general roads musings.
There is a separate forum for Street Furniture (traffic lights, street lights, road signs etc).
Registered users get access to other forums including discussions about other forms of transport, driving, fantasy roads and wishlists, and roads quizzes.
jackal wrote:Here's my go at the Wisley upgrade.
Wisley.png
And a coloured version so you can see which ramp's doing what a bit easier.
Wisley coloured.png
The main consideration was ensuring that all movements could stay functional during construction, which means the roundabout stays in the middle until it's no longer needed.
Construction phases as follows:
1. All left turns constructed. Blue and green right turns constructed over/under on a fourth level. Thus, six of eight turns are off the roundabout.
2. Two new bridges adjacent to the roundabout are built over the M25 for yellow and red right turns, along with associated links.
3. The yellow and red routes are completed by using the existing north and south roundabout underpasses.
4. The east and west roundabout bridges are demolished.
Great stackapool!
Could use existing M25 bridges on red and yellow routes to save money though.
The problem now is IF they want to improve this junction the window of opportunity is disappearing fast.
Assuming that Heathrow's third runway goes ahead that will make such a mess of M25 traffic flows for years that no major works will be allowed on the rest of the route from the M23 around to the west to at least the M1. Wisley's mess just won't be allowed.
Will be interesting to see the proposals. I can't quite shake the fear that they were joking about freeflow and will manage to spend the £250m on a 20-phase double hamburger or something...
Glom wrote:Can't see how those gradients work on the semi-directional slips.
The same way they do on my earlier design (yellow and red slips below). The two new bridges over the M25 aren't flat like the existing roundabout bridges, they increase elevation. They also cross the offslips away from the roundabout, where the offslips are a bit lower (because they are coming up from a lower level). Combining those two factors is plenty to get the necessary difference in vertical alignment.
Looks like the design tries too hard to reuse the existing roundabout structures.
You could just build stack-style connectors that don't cross each other on the middle level but it would be very difficult to do that while maintaining use of the roundabout during construction. It would mean you are building two massive skewed bridges inside the roundabout, at the same level, which I don't think there is really space for, and would be very disruptive even were it possible.
My solution provides an equally high standard of connector and requires much less dramatic traffic management, because the new structures aren't as large and are on the outside rather than inside the roundabout. It's very common to have connectors crossing as my red and yellow ones do, and there's nothing especially difficult about it - a turbine/whirlpool actually does that with all its right turns.
It's another site I can see a bunfight over given it's heavily wooded surroundings. Dare I say any improvements will try and keep the overall height of the junction as low as practical, which probably means any free-flow designs will be mostly offline and potentially in tunnels or cuttings?
Bryn Terminally cynical, unimpressed, and nearly Middle Age already. She said life was like a motorway; dull, grey, and long.
Bryn666 wrote:It's another site I can see a bunfight over given it's heavily wooded surroundings. Dare I say any improvements will try and keep the overall height of the junction as low as practical, which probably means any free-flow designs will be mostly offline and potentially in tunnels or cuttings?
I dunno, it's already three levels, which is enough for any freeflow design except a full stack. Given the way the last two major stackabout upgrades went (M25/A12 and M62/M57) and the potential for cost savings, I'm expecting a loop-based design, which does at least keep the overall level low. You can still have woodland inside the loops with tunnels for wildlife etc.
I would think a cloverstack would work will the semi-directional slips being marginally further out than the roundabout, keeping than in place until it can be demolished once complete.
Glom wrote:I would think a cloverstack would work will the semi-directional slips being marginally further out than the roundabout, keeping than in place until it can be demolished once complete.
That would basically mean going through the opposite sides of thesewalls. You'd have to build new highly skewed underpasses with retaining walls and raise the level of the A3 to achieve the necessary vertical alignment. I think it would be more expensive and disruptive than a design that reuses the existing underpasses, like my cloverturbine above.
That said, what you're describing would actually work and it may be I'm underestimating the challenges involved in my design. In truth, HE will probably come up with something completely different. The only full stackabout rebuild I'm aware of, which was in Lummen, Belgium, created a full turbine/whirlpool, so that's one credible if expensive option.
Interesting that all suggestions are pretty symmetrical. My thoughts are that there will be an incremental approach to arriving at the design solution. Firstly understand the dominant movements (IMV M25 ACW to A3 N and vice versa) and once these are made freeflowing look again at the remaining junction and see how it works with the movements removed. It may well be that, along with appropriate widening of the A3, the capacity released on the circulatory may be sufficient to give a reasonable design life for the junction.
IIRC this was the approach at M25 junction 2 where I think there were two freeflow viaducts originally proposed to and from both directions of the A2 but on analysis only one was needed.
M25 Junction 2 is still something of a bottleneck at peak times for turning traffic though. Yes, movements have been taken out of the roundabout, but there are still high volumes of conflicting turning movements. The prime example is A2 from London to M25 heading south, versus M25 from the south to A2 heading towards the Medway towns. And M25 Junction 2 was a very structurally significant and expensive way of providing just a single freeflow movement compared with what is possible at any three level stacked roundabout. How many bridges/viaducts are there at Junction 2 totalling what cumulative length? I imagine it is more than most fully freeflow interchanges on the M25.
RichardA35 wrote:Interesting that all suggestions are pretty symmetrical. My thoughts are that there will be an incremental approach to arriving at the design solution. Firstly understand the dominant movements (IMV M25 ACW to A3 N and vice versa) and once these are made freeflowing look again at the remaining junction and see how it works with the movements removed. It may well be that, along with appropriate widening of the A3, the capacity released on the circulatory may be sufficient to give a reasonable design life for the junction.
IIRC this was the approach at M25 junction 2 where I think there were two freeflow viaducts originally proposed to and from both directions of the A2 but on analysis only one was needed.
Taking a closer look at the 'background' tab of the scheme page, I am concerned that the language seems to imply the continued existence of the roundabout:
The key benefits of this project are: increased road capacity at the M25 junction10 roundabout
increased road capacity on the A3 between Ockham and Painshill
improved traffic flow and reduced delays at M25 junction 10 and on the A3 improved safety on the A3, its entry and exit roads and the M25 junction 10 roundabout reduced queuing as traffic enters the M25 junction 10 roundabout
improved access to RHS Garden, Wisley
This seems to support the idea of a more incremental upgrade as you suggest, which would be a terrible shame given the failure of expensive partial upgrades at sites such as Lofthouse and M25 J2. On the other hand, the scheme taken to consultation sometimes has little relation to the scheme page description (e.g. M25 J28), so maybe they will go for 'free-flowing movement in all directions' as specified in the RIS.
guvvaA303 wrote:What we have heard is that 2 options are being looked at in Guildford, an extra lane or a complete new bypass with a tunnel under Hogs Back.
Adding an extra lane up to the A31 Hog's Back junction seems the better option to me. Plus closing the at grade crossings between there and the Stag Hill/Cathedral junction, preferably by giving Beechcroft Drive an over/underpass.
A tunnel would be very expensive and as we've seen with the Hindhead Tunnel liable to frequent closures. Plus I think a lot of the traffic using the A3 through Guildford is probably headed to or from the town itself rather than wanting to bypass it completely.
As somebody who often uses the Guildford bypass, I am not convinced about A3 traffic through Guildford being traffic that is trying to get to or from Guildford. True, a lot of traffic enters the A3 on the London side of Guildford, which is why the A3 goes from D2 to D3 at that point. However the holdups are on the bypass itself, often caused by a relatively small amount of traffic trying to join the A3.
I've lost track of this thread, but thinking about junctions like M25 j2 where a stackabout was partially upgraded by building a quarter of a stack to handle what is judged to be the most critical movement, is it feasible to build the opposing quarter and then use the roundabout structures for the remaining two by simply routing the right turning traffic the wrong way round so there is no conflict?
