Bump!
jackal wrote: ↑Thu Apr 14, 2022 16:33
Peter Freeman wrote: ↑Thu Apr 14, 2022 05:19
I like Jackal's GSJ hierarchy (subject always of course to local considerations) ...
... a
signalised diamond can handle far more traffic than a dumbbell - it's a function of how wide you flare the off ramps at the signal line (simplification).
... While a signalised diamond could have more capacity than a dumbbell, I wouldn't really consider such a junction to be in the modern hierarchy. It's basically superseded by the DDI and SPUI, much like the two-bridge roundabout - which doesn't stop them getting built, of course.
My current version of the motorway interchange competence table is as follows -
1. Diamond (unsignalised)
2. Dumbbell
3. Diamond (signalised)
4. Parclo five-ramp (one loop)
5. DDI
6. SPUI
7. Parclo six-ramp (two loops)
8. Full freeflow
That's Jackal's list, but with two additions by me: signalised diamond in position 3 and five-ramp parclo in position 4. It's a USA or international list - the UK version is rather different.
Reviewing the discussion from a couple of years ago, my recent thoughts, with an AU bias, are -
1.
Unsignalised diamonds are only suitable for motorway interchanges with minor rural roads. Perhaps
dumbbells also, soon.
2. The
signalised diamond appears to still be the default non-freeflow GSJ in Australia, and, I believe, in Canada. Possibly most new builds in USA are DDI - I'm not sure. Therefore it remains in the modern hierarchy. It can have very high capacity. AU has many. Here are examples that are topical at present -
a) A new signalised diamond recently replaced an incomplete half dumbbell here that was originally intended to be completed as a full dumbbell. Clearly, the tide in AU has turned against urban dumbbells -
https://www.google.co.uk/maps/place/Pak ... ?entry=ttu
b) The dumbbell here is also currently being replaced. A DDI would fit, but actually it will get a large signalised diamond. Why? Because (my guess) although it will initially have a high west-to-south flow, that movement will in future be carried by a planned motorway-grade corner-cut, leaving this interchange as more straight-through -
https://www.google.co.uk/maps/place/Pak ... ?entry=ttu
c) This struggling signalised diamond will soon be widened, but with no change to its form. Why? Probably not enough space on its south side for a DDI -
https://www.google.co.uk/maps/place/Pak ... ?entry=ttu
3. The above three examples are all on one stretch of Melbourne's M1, built fifteen years ago with five consecutive dumbbells. It's obvious now that they will all eventually be replaced. This echoes what is happening in other AU cities.
4. The
DDI marches forward, especially in the USA where about 160 are in operation. AU appears to be in distant second place with 3 built and 4 more committed, and Canada third with 3 built. Its strengths: handling high turning volumes, low cost, and it's an easy upgrade for a struggling diamond.
5. The
SPUI continues to reign for heavily-trafficked urban new-builds. Its strength: dynamic adaptability to unbalanced high flows when signalised in four stages. USA has many, AU has about 40.
6. In AU, both DDI and SPUI are usually fully signalised, even the four left turns.
7. The
six-ramp parclo is the about the closest you can get to full freeflow, without actually being full freeflow. It's very useful where a busy arterial road crosses a motorway at a skew angle. That generates high volume right turns in opposite quadrants, which is exactly where this type's 270 degree loops are situated. Thus, two right turns become freeflow. The other two rights, and the straight-throughs, remain signalised but only lightly. Here are two of the three that exist in Perth AU -
a) This one is classic layout and provides two freeflow right turns -
https://www.google.com.au/maps/place/Pe ... ?entry=ttu
b) This one is six-ramp but not classic layout because too much other stuff is attached, and so it doesn't quite provide freeflow rights -
https://www.google.com.au/maps/place/Pe ... ?entry=ttu
8. In a
five-ramp parclo only one of the four right turns is a loop. This does not produce a freeflow right, but it means that the looped right turn needs to pass through only one signal, instead of the two in a diamond. This suits some situations where one turn dominates. Here are two of the several that exist in Perth -
a) Here's a classic example -
https://www.google.com.au/maps/place/Pe ... ?entry=ttu
b) And here's an impure form, since one loop is a genuine right turn only but the other functions also as a folded left turn. It still does allow two right turns to encounter only one signal -
https://www.google.com.au/maps/place/Pe ... ?entry=ttu
9. Parclo's can be drawn in many different forms. The loops may be for on-ramps or off-ramps, depending which dominant turns need to be favoured. Some, with subsidiary roads thrown into the mix, become complicated, while some are so simple (four ramps) that they are actually just folded diamonds. The 5-ramp and 6-ramp types well deserve their places in our list.
10. The main point of this rather long post is that in AU the most common motorway service interchange type is diamond, usually signalised. It's our equivalent of UK's GSR. I think this will continue, though I'd love to see the DDI take over.
(Edit: trivial word-smithing)