Introducing the Sparclo, an SPUI/Parclo hybrid (with mocked up US example)

[jackal]

Yes Chris, SPUI-ization of an existing 6-ramp parclo might not produce great gains because, at most of them, the hooked long turns are lightly loaded. Light loading of the two at-grade long turns especially occurs where two main roads cross at a skew, which is one of the best reasons for selecting a 6-ramp parclo (as at Tonkin Hwy, Perth WA, Jackal). With light loading, the usual problem (a green phase runs out of stored vehicles, as at a standard diamond service interchange) doesn't occur. And, as you surmise, Chris, the two at-grade long-turn greens can occur simultaneously, so there can be 2-stage operation, and so single-point provides little gain.

One point I would mention is that the "reservoir" isn't only filling up from long turns, but also from straight-through traffic on the surface street. So even if the two non-loop long turns are relatively light, it could be that the reservoir capacity is exceeded if the street has high volumes. The sparclo would be helpful in such cases.

"...but also from straight-through traffic on the surface street ..."
I'm not sure that I understand your argument here.

Let's assume (in order to keep the discussion simple) that the standard 6RP is in the UK and is the way around where a N-S motorway feeds the loops via its exit ramps. These free-flow long turns never stop, so neither of them is filling up a reservoir. The reservoir consists of only the right-hand lanes of the cross street between the two at-grade intersections. Straight-through E-W and W-E vehicles must stop occasionally for turning traffic to cross right-to-left at-grade, but not for long, and they do not feed the reservoir: they subsequently drive past red right arrows. The only source of vehicles into the reservoirs is E-W and W-E traffic wanting to turn right. Typically, neither of these movements will be high flow, as appropriate design will cause loops to serve the high flows.

This is why I think the Sparclo is a good idea, but, in most cases, will produce only a moderate capacity increase.

This made no sense to me until I realized you might have in mind the Perth interchange we've discussed several times, which does indeed stop the surface street only once at lights, with the through route essentially bypassing to the left of the second set of lights: https://www.google.co.uk/maps/@-31.7956 ... ?entry=ttu

The first thing to be said, then, is that this is really a very unusual (and good!) arrangement. The vast majority of 6RPs worldwide stop the surface street through traffic twice. Here are three examples for each of Florida, California and Ontario, though scores of cases could be provided:

Florida (freeflow):
https://www.google.co.uk/maps/@25.80937 ... ?entry=ttu
https://www.google.co.uk/maps/@27.41676 ... ?entry=ttu
https://www.google.co.uk/maps/@26.49343 ... ?entry=ttu

California (non-freeflow):
https://www.google.co.uk/maps/@33.92538 ... ?entry=ttu
https://www.google.co.uk/maps/@33.67697 ... ?entry=ttu
https://www.google.co.uk/maps/@37.38564 ... ?entry=ttu

Ontario (non-freeflow):
https://www.google.co.uk/maps/@43.79297 ... ?entry=ttu
https://www.google.co.uk/maps/@43.83810 ... ?entry=ttu
https://www.google.co.uk/maps/@43.62067 ... ?entry=ttu

Compared to this dominant configuration surely the sparclo has a great advantage, because it helps through traffic a lot as well as turning traffic.

There's the remaining issue of the comparison between the sparclo and the Perth-style 6RP. Here I agree the sparclo's advantage is smaller, as it's just the turning traffic that benefits. Still, the advantage is there. I also suspect that the Perth 6RP has a slightly enlarged land take much like the sparclo, as it's advisable to somewhat separate the right-turning traffic from the fast moving traffic to its left. They are both slightly more complex versions of the usual 6RP, which basically just has a square of signalised junction at each end that you can't bypass.

I should also mention an unusual variant on the Ronald Reagan Turnpike in Miami, which is the "other way round" from the Perth example, i.e., the surface street feeds the loops, as is common in Florida. What's unusual about it is it has a freeflow bypass of the first set of lights for westbound SW 88th St traffic: https://www.google.co.uk/maps/@25.68631 ... ?entry=ttu It's basically the equivalent of the Perth 6RP for where the loops are the other way round. Again, there is some extra land take, and there would be much more if it were built to modern engineering standards, I expect. (A couple of complications: a bypass is only provided westbound, so eastbound has to stop at both lights, and there were works at the time of the satellite imagery to make the westbound loop freeflow, which are complete on GSV, with the bypass remaining.)

[Peter Freeman]

We need a UK location that would be suitable for this challenge I think.

It depends on what you want to draw -
a) Convert SPUI to Sparclo
b) Convert 6RP to Sparclo
c) Design new-build Sparclo.

Option (a) is impossible and irrelevant for UK, as there is no standard SPUI.
Option (b) could use a UK example (was there one?) out of Jackal's recent 5-6-7 Parclo topic.
Option (c) would perhaps be best, especially if you could incorporate my 'displaced single-point' idea!

[jackal]

We need a UK location that would be suitable for this challenge I think.

It depends on what you want to draw -
a) Convert SPUI to Sparclo
b) Convert 6RP to Sparclo
c) Design new-build Sparclo.

Option (a) is impossible and irrelevant for UK, as there is no standard SPUI.
Option (b) could use a UK example (was there one?) out of Jackal's recent 5-6-7 Parclo topic.
Option (c) would perhaps be best, especially if you could incorporate my 'displaced single-point' idea!

Re: (b), Sevenoaks is a standard 6RP but not convertible to sparclo due to its tightness and high skew.

Pathing the SPUI-style slips in an existing 6RP is awkward, as Peter says. The problem is that you want a squarish box like an SPUI but the box between the loops on a 6RP is usually quite an elongated rectangle, which would mean the turning traffic would be virtually driving along the surface street, taking a long time to clear. This is why I didn't mention upgrading an existing 6RP as a particular strength of the design, though there are some cases where the loops are close to each other (the Perth example is one, though it doesn't need the upgrade) or where there is spare space (either for Peter's displaced single-point or simply to run the slips further into the junction before they cross).

One idea for a sparclo in the UK would be to take an overloaded two-bridge roundabout, use the existing bridges for the slips going to or from the loops (similar to how I used the extra spans in the OP drawing), and build a new SPUI-style bridge in the middle.

[Peter Freeman]

I didn't realise that the Perth 6RP was so unusual in having that 'bypass'. I was aware that many parclo's enforce a stop where they don't really need to (including many folded diamonds in AU), but with 6RPs I had just assumed a fairly even split! The proposed new one on Tonkin Hwy south will be similar to the northern one.

Miami: yes, similar in principle - on a Miami scale! G.Earth-historical's latest image shows completion. I drove through Miami (to the Beach) once - so impressive!

[Peter Freeman]

One idea for an SPUI in the UK would be to take an overloaded two-bridge roundabout, use the existing bridges for the slips going to or from the loops (similar to how I used the extra spans in the OP drawing), and build a new SPUI-style bridge in the middle.

There are so many innovative ways, some at quite low cost, that an overloaded 2-bridge GSR could be re-worked. Your idea is one, and a good one, where there is space. I'm sure that some, with a thin elongated roundabout shape, could easily make a DDI. Others could become a generous but standard SPUI by 'planking over the hole' - as was partially done at M6J19 and as Bryn drew up for M67/M60.

You can, of course, simply make the roundabout bigger (M6J10, M6J11, M25J10, ...) !!!!!!!!

[Peter Freeman]

Compared to this dominant configuration surely the sparclo has a great advantage, because it helps through traffic a lot as well as turning traffic.

Yes. But another conclusion is that the most cost-effective way to enhance an existing 6RP that has those straight-through signals, is to remove them and install mildly-segregated right turn arrow signals.

There's the remaining issue of the comparison between the sparclo and the Perth-style 6RP. Here I agree the sparclo's advantage is smaller, as it's just the turning traffic that benefits. Still, the advantage is there.

Yes, but a very minor advantage.
Sparclo: straight-through E-W traffic will stop only once, briefly, at the crossover, if they're unlucky. Right-turners will stop only once, for a longer period, at the crossover, unless they're lucky.
Perth 6RP: straight-through E-W traffic will stop only once, briefly, at the first signal, if they're unlucky. Right-turners will stop once, briefly, at the first signal, if they're unlucky; and then, for a longer period, at the red arrow, unless they're lucky.

On much reflection, I think the cost of reconfiguration of either a standard or Perth-style 6RP to Sparclo could not be justified (and, I know, you did not promote this). Conversion SPUI to Sparclo: maybe. New-build Sparclo: maybe, but even there the extra space might be better used for longer/more lanes.

BTW, the Perth 6RP has a wide unused space between the E-W carriageways (under the bridge) between the at-grade intersections. I wonder why ...

[Peter Freeman]

... It's interesting, to me at least, that seemingly completely different designs have quite a lot in common when you look under the bonnet.

Interesting to me too. A bit like 'morphing' one type to another merely by pulling a connector into a different quadrant.

Thinking further on this, I wondered what Jackal's thought process was when he invented the 'Sparco'. I suppose it was simply " If a single point crossover can solve the standard diamond's problem of opposing right turns hooking around each other, perhaps it can solve the 6RP's similar problem ".

A different thought process might be:
" Here I have a 6RP, with the two at-grade right turns well-separated. That's good, because the reservoir length is long enough and capacious enough that straight-through traffic isn't stopped too frequently. I wonder what would happen if I had to move them closer together (eg. a pond full of Lesser Crested English Newts is in the way)? Oh, still OK, but only just. The reservoir has become really small, making the signal cycle rapid and inefficient. Bad news then arrives: an Aboriginal sacred tree has been found growing on the other side of the motorway, so I'll have to push them even closer, dammit. Here we go then. Oops, I've pushed the right turns together so far that they've just passed each other! Oh - wait a minute - the reservoir problem's gone - I've invented a Sparclo! ".

[Peter Freeman]

... Miami: yes, similar in principle - on a Miami scale! G.Earth-historical's latest image shows completion. ...

Additional comments -

The Miami 6RP is very well done, and nicely improved. It's a pity they couldn't fit a bypass for eastbound straight-through too.

All your listed examples not only have that straight-through unnecessary stop, but are also the 'other way round' to Perth: their loops are motorway on-ramps, and the unnecessary stop is their first stop instead of second (in Perth). This makes the elimination of straight through signals more difficult to lay out. Miami does it with a separate bypass carriageway. Also, in many locations, pedestrian requirements might disallow the signals removal anyway.

[jackal]

... It's interesting, to me at least, that seemingly completely different designs have quite a lot in common when you look under the bonnet.

Interesting to me too. A bit like 'morphing' one type to another merely by pulling a connector into a different quadrant.

Thinking further on this, I wondered what Jackal's thought process was when he invented the 'Sparco'. I suppose it was simply " If a single point crossover can solve the standard diamond's problem of opposing right turns hooking around each other, perhaps it can solve the 6RP's similar problem ".

A different thought process might be:
" Here I have a 6RP, with the two at-grade right turns well-separated. That's good, because the reservoir length is long enough and capacious enough that straight-through traffic isn't stopped too frequently. I wonder what would happen if I had to move them closer together (eg. a pond full of Lesser Crested English Newts is in the way)? Oh, still OK, but only just. The reservoir has become really small, making the signal cycle rapid and inefficient. Bad news then arrives: an Aboriginal sacred tree has been found growing on the other side of the motorway, so I'll have to push them even closer, dammit. Here we go then. Oops, I've pushed the right turns together so far that they've just passed each other! Oh - wait a minute - the reservoir problem's gone - I've invented a Sparclo! ".

Haha! It was definitely more like the first. I was basically inspired by the stacked SPUI you've been promoting and the dumbbell-DDI I mentioned before to think about which elements of different junctions could be fruitfully combined. I think I started off thinking about DDI hybrids, e.g., a stacked DDI, but it didn't really make sense structurally as you'd have a middle level elongated above a mainline. I moved on to thinking about SPUI hybrids and found one that worked.

Additional comments -

The Miami 6RP is very well done, and nicely improved. It's a pity they couldn't fit a bypass for eastbound straight-through too.

All your listed examples not only have that straight-through unnecessary stop, but are also the 'other way round' to Perth: their loops are motorway on-ramps, and the unnecessary stop is their first stop instead of second (in Perth). This makes the elimination of straight through signals more difficult to lay out. Miami does it with a separate bypass carriageway. Also, in many locations, pedestrian requirements might disallow the signals removal anyway.

Yes, as I mentioned before, it is considered an important aspect of design on Ontario to have traffic coming off the highway stopping at signals. You'll notice this applies to the short turns as well, i.e., the Ontario examples have freeflow rights onto but not off the highway. They have this one specific configuration they build over and over again.

Elsewhere you can find 6RPs built the Perth way round, e.g., this in Florida: https://www.google.co.uk/maps/@27.44689 ... 393175,16z My hunch is this configuration tends to coincide with a low priority for pedestrians, due to the aforementioned issue of high speeds on ramps coming off the motorway. Hence it may be more likely in less active travel-leaning jurisdictions and/or more more rural locations.

[Peter Freeman]

Yes, as I mentioned before, it is considered an important aspect of design on Ontario to have traffic coming off the highway stopping at signals.

I agree with Ontario's design guideline. Considering the long-term, I think Perth would have been wiser to comply.

You'll notice this applies to the short turns as well, i.e., the Ontario examples have freeflow rights onto but not off the highway.

Perth too: left turns onto Gnangara Road are not freeflow - which you once commented on.

... Elsewhere you can find 6RPs built the Perth way round, e.g., this in Florida: https://www.google.co.uk/maps/@27.44689 ... 393175,16z My hunch is this configuration tends to coincide with a low priority for pedestrians, due to the aforementioned issue of high speeds on ramps coming off the motorway. Hence it may be more likely in less active travel-leaning jurisdictions and/or more more rural locations.

This might have been Perth's thinking. Although Perth is an active-travel-leaning jurisdiction, both ends of Tonkin Hwy are far out and lowly-populated *. However, the city is growing rapidly, so these interchanges may well find themselves eventually surrounded by suburbia. The need may arise then to add extra nmu crossing facilities, perhaps grade-separated, at both **.

* Gnangara Road, and Thomas Road to a much lesser degree, have the look of being destined to become important E-W corridors. Gnangara might develop into a semi-motorway.

** Perth's extensive 'principal shared path' (PSP) network reaches both of these locations, and already incorporates a full-span bridge over Gnangara Road (east), eliminating two high-speed ramp crossings.