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=ttuPeter Freeman wrote: ↑Sat Jan 13, 2024 06:25"...but also from straight-through traffic on the surface street ..."jackal wrote: ↑Wed Jan 10, 2024 15:35One 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.Peter Freeman wrote: ↑Wed Jan 10, 2024 11:36
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.
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.
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.)