Australian Motorways

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jackal
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Re: Australian Motorways

Post by jackal »

Peter Freeman wrote: Sun Apr 17, 2022 12:54 ^ I've always been amazed by this curious fact too. I don't think it's deliberate avoidance. Rather it's just the way it has worked out.

This tangled mess in Brisbane perhaps counts - the M3 Inner City Bypass (ICB) crosses the M7, which simultaneously splits into the M7 airport tunnel and A3 surface. Here (but better viewed on GE) -
https://www.google.com.au/maps/@-27.444 ... 658,16.11z

Also in Brisbane: follow the Logan Motorway eastwards to its merge with M1. That will become a 4-way when the Coomera Connector Motorway continues the M6 (or M2?) southwards to the Gold Coast. The D3 CCM will run parallel to the M1, which is already bursting its D4 and D5.

A place where Melbourne could have used a 4-way is M1/M3. Now there's insufficient space. There would be M3 northbound to M1 eastbound traffic demand and the reverse if provision was there. Currently there are adequate non-freeway alternatives, and a freeway-based alternative route will eventually arise from planned upgrades SE of there.

Melbourne tentatively plans for three interchanges on its Outer Ring to be stacks, but that project is far in the future. A 4-way is planned at M3/Dandenong-Bypass, when the latter receives its upgrade to motorway. Perhaps a cloverstack. Timing: probably far off.

I'm trying to think of the other extant one you mention. What's an 'offside stack'?
Taking each paragraph in turn:

The M1/M3 one is the main example where it almost looks deliberate (more on this below).

I can't see ICB east to M7 north. Like M1/M3, this is one of several cases in Australia where 6 or 7 out of 8 freeflow turns are provided (with only one of the right turns missing). Some others are M2/M80 (Melbourne), Glenloch Interchange (Canberra), and the new M2/A9 interchange (Adelaide). The only 'three quarter access' freeflow interchange like this in the UK I can think of is A739/A814 in Glasgow, which is a bit of a cheat as it relies on weaving and a U-turn (M60/A34 is nearly there but some of the turns are actually to/from M56 only). To an extent one might say that in Aus these are filling the role usually performed by four-way full access interchanges.

I guess the four way M1/M6 would be another example of three quarter access, as there'd be little point putting links between the M1 south and M6 south.

It would not have cost a lot to put in a loop for M3 south to M1 east and the corresponding left turn. I suppose it was built long enough ago that the demand wasn't obvious.

At least the futureproofing is there for this one.

An offside stack has at least one merge or diverge to/from the offside (right in UK/Aus), while having a generally stack-like arrangement (i.e., no loops, the carriageways don't cross over like a gothic interchange, nor are both carriageways splayed to facilitate long turns, i.e., right in UK/Aus). This still encompasses a wide range of designs. The only such interchange in Western Europe is this outside Zurich: https://www.google.com.au/maps/@47.4119 ... 275,16.72z They are more common in the US. Here are a couple of examples in Connecticut: https://www.google.com.au/maps/@41.3007 ... 111,16.31z and https://www.google.com.au/maps/@41.7823 ... 697,14.63z (Obviously, because they have offside turns, they are never going to be completely stack-like, but there is a resemblance, especially with the use of direct connectors for long turns.)
Peter Freeman wrote: Sun Apr 17, 2022 14:22
jackal wrote: Sun Apr 17, 2022 11:05 Anyway, this quirk is gradually being eroded, with the latest additions being cloverstacks outside Brisbane and Perth:
https://www.google.com.au/maps/@-27.645 ... 429746,16z
https://www.google.com.au/maps/@-31.863 ... 210302,16z
Jackal, did you notice, on the Brisbane cloverstack -
(a) The metering signals on a connector and a mainline? It solves the problem of only 400m spacing between an entry and exit, southbound.
(b) The RHS exit in the stack, southbound?

BTW, this interchange has been an almost complete cloverleaf (but with one signalised movement) since 2003. You're right though that it has only recently become full freeflow (not counting the metering .... :?).
Hadn't noticed that - yes, technically not freeflow, like the full stack in Athens with toll booths. Seems silly to go to all that effort and then signalise it for the sake of a local exit. This is what C/D roads and braiding are for (even more effort, I know). And yes, that issue aside, it's an offside cloverstack in my accounting.
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Re: Australian Motorways

Post by Peter Freeman »

^
Brisbane: you're right, M7/ICB is not complete. Adding that nmissing turn would not be justifiable. Or perhaps they ran out of spaghetti.

Brisbane: I haven't seen a design for the future M1/M6, but you're right that it's most unlikely to be 8/8. (btw, I think Coomera Connector is actually going to be M2 not M6, but that will require a slight juggle of the current designations as Logan Motorway at present morphs invisibly into M6 before reaching M1. Coomera Connector stage 1 is now under construction, concurrently with Pacific Motorway still receiving billions of $$ in upgrades).

Brisbane: Mount Lindesay Road is generally not quite motorway (though, if you follow southwards from M2 for a few km, on recent Google Earth images, you'll see that it has some interesting interchanges and service roads). There's a 10-year plan for further D2 and D3 upgrades, extending quite far southwards, but with occasional major signalised intersections interspersed with the GSJ's. So, that signalised merge near M2 instead of braiding is not out of place. Actually, it's surprising that that location even merited a cloverleaf or cloverstack in the first place. Starting now it would probably be a SPUI.

Melbourne: M3 S to M1 E missing both directions: yes, there might just be space for a loop (it's tight, some bridging required, and there are level problems), but the left turn won't fit.

More generally:

1. Your contention that our number of 8/8's is very low compared with elsewhere does seem to be correct, and outside of the 'just-by-chance' probability. So perhaps it is deliberate, or reflects a different philosophy. You will be aware that AU cities have wide at-grade roads that are additional to, and generally preceded, the motorway networks. These have frequent signals, but carry large traffic volumes. In Melbourne, good examples are Nepean Hwy and Dandenong Road (both in some places are D4 with 2-lane 1-way service roads on each side), and Maroondah Hwy east of Ringwood. To let them substitute for some missing freeway turns is therefore a realistic and pragmatic option. UK cities have fewer such strong wide roads, so missing motorway turns are more detrimental.

2. Regarding those metering signals in Brisbane, I suppose AU drivers will find them acceptable since
(a) we're getting used to ramp metering, and connector metering,
(b) we have far more signals than roundabouts (in other words, we're used to stopping!), and
(c) our road categories are less well differentiated than in many other countries. For example, our rural motorways have occasional sub-standard features and interchanges.

To me, pragmatically (as long as safely) including traffic signals on a motorway interchange is not a problem (even though, like Truvelo, I prefer my GSJ's big and complex).

edits:
1. Added comments about Mt Lindesay Road
2. Corrected 'S2 service lanes' to '2-lane 1-way service roads'
3. Added "and connector metering"
Last edited by Peter Freeman on Fri Jun 07, 2024 12:25, edited 4 times in total.
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Re: Australian Motorways

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My mystery 'offside stack' was probably an incorrect entry for the offside cloverstack outside Perth airport, which I did know about but had forgotten. This was constructed in late 2015. So that would be four four-way full freeflow interchanges in Aus, or three if we disqualify the Brisbane one with its signals.

In my efforts to find the mystery interchange I noticed a few more three-quarter freeflow interchanges:

Perth: https://www.google.co.uk/maps/@-32.0867 ... 508,15.84z
Brisbane: https://www.google.co.uk/maps/@-27.6257 ... 0039,15.3z
Brisbane: https://www.google.co.uk/maps/@-27.5748 ... 5371,15.3z
Newcastle: https://www.google.co.uk/maps/@-32.8884 ... 8985,15.3z

Really a lot of these compared to the four way full freeflow. Must be ten or more? Though also it strikes me that the UK has an unusually small number of this type (can there really only be one?).
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Re: Australian Motorways

Post by Peter Freeman »

jackal wrote: Sun Apr 17, 2022 18:00 An offside stack has at least one merge or diverge to/from the offside (right in UK/Aus), while having a generally stack-like arrangement (i.e., no loops, the carriageways don't cross over like a gothic interchange, nor are both carriageways splayed to facilitate long turns, i.e., right in UK/Aus). This still encompasses a wide range of designs. The only such interchange in Western Europe is this outside Zurich: https://www.google.com.au/maps/@47.4119 ... 275,16.72z They are more common in the US. Here are a couple of examples in Connecticut: https://www.google.com.au/maps/@41.3007 ... 111,16.31z and https://www.google.com.au/maps/@41.7823 ... 697,14.63z
Does the second American one really count? It looks more like two adjacent T's on the I84 that overlap. However, since you don't have to join the I84 carriageways, you could say it amounts to a 4-way (a concept that we've aired previously).
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Re: Australian Motorways

Post by jackal »

Well, a stack basically is two Ts with an extra bit of mainline between them. This is perhaps more obvious for the Houston configuration: https://www.google.co.uk/maps/@29.78473 ... 576,16.47z The Connecticut example is just an elongated version of the same (plus offside entries/exits).
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Re: Australian Motorways

Post by Peter Freeman »

jackal wrote: Tue Apr 19, 2022 12:35 Well, a stack basically is two Ts with an extra bit of mainline between them. This is perhaps more obvious for the Houston configuration: https://www.google.co.uk/maps/@29.78473 ... 576,16.47z The Connecticut example is just an elongated version of the same (plus offside entries/exits).
Indeed. It is quite fascinating to stretch and bend such layouts to where they are almost unrecognisable, without affecting the underlying topology, or requiring extra bridges, or changing the sense of any merge or diverge.

In the same way that two opposing or adjacent motorway T's can make a 4-way cross, two opposing T's on a normal road make a normal 'crossroads'. If you pull the opposing roads apart a little way, it makes a 'staggered crossroads'. We sometimes do this, for low traffic, to improve safety. However, if we pulled them very far apart, we'd cease to call it a crossroads and just consider them to be two flat T-junctions.

In that I84 example, the opposing freeways have 2.5 km of separation. I'd call it a marginal case. Much more and we wouldn't count it as a 4-way cross. Especially as the opposing arms are numbered I291 and I384. And changing one number so they're both either 291 or 384 would be non-compliant with the Interstate network's numbering system.
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Re: Australian Motorways

Post by Glenn A »

I was in Perth in 2006 and remember the freeway crossing the city from north to south and by passing the city centre. Also the speed limit was quite generous at 120 km/h, and on the Highway 1 to Bunbury, the speed limit on the S2 sections being 110 km/h. close to the old 70 mph limit on S2s over here from before 1974.
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Re: Australian Motorways

Post by Peter Freeman »

Adelaide, state capital of South Australia, is now set to join Australia's 'underground motorways club'. One of the city's few motorways, the quite lengthy M2 North-South Motorway, has a gap at its busiest part as it bypasses the CBD. Construction is soon to begin on its completion by two consecutive twin tunnels. Information is here -
https://dit.sa.gov.au/nsc

Sydney is AU's best example of going underground to avert surface mayhem and nimbyism associated with new urban roads. It might be, or soon become, the world's leader in this trend.

Not too far behind is Brisbane. Here's an interesting photo that shows where two of its (independent) tunnels begin, side-by-side. On the left, M7; on the right, M3. If the photographer turned around, the view would be of another portal: the Airport Link Motorway. Photo location -
https://www.google.com.au/maps/@-27.443 ... 325951,15z

M7+M3 Tunnels.jpg
(edited to update project website URL)
Last edited by Peter Freeman on Thu Jul 06, 2023 07:44, edited 1 time in total.
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Re: Australian Motorways

Post by Peter Freeman »

While on the subject of placing urban motorways underground, yesterday, not far from my home, I walked by the Victorian Tunnelling Centre. This is an institution that opened about two years ago in anticipation of tunnelling work beginning for the Westgate Tunnel Project in Melbourne. Inspired by Crosslink's 'Tunneling and Underground Construction Academy'. Its purpose is to train engineers, and especially technical crews and drivers, in the operation of equipment such as TBMs and roadheaders. The yellow item in the photo is an actual (used and retired) cutter head from a local TBM.

https://bigbuild.vic.gov.au/projects/no ... ing-centre

victunnelcentre_1778.jpg

[edit: link added]
Last edited by Peter Freeman on Wed Feb 08, 2023 09:53, edited 1 time in total.
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Re: Australian Motorways

Post by Peter Freeman »

jackal wrote: Sun Apr 17, 2022 11:05 I have previously mentioned the extraordinarily low number of four way, full access freeflow interchanges in Australia ...
Anyway, this quirk is gradually being eroded ...
Here's another Australian 8/8, in Perth -
https://www.google.com.au/maps/@-31.964 ... 146,15.04z
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Re: Australian Motorways

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Peter Freeman wrote: Wed Aug 31, 2022 08:00
jackal wrote: Sun Apr 17, 2022 11:05 I have previously mentioned the extraordinarily low number of four way, full access freeflow interchanges in Australia ...
Anyway, this quirk is gradually being eroded ...
Here's another Australian 8/8, in Perth -
https://www.google.com.au/maps/@-31.964 ... 146,15.04z
Mentioned in my post of 19 April above.

A couple of junctions to the north is this nice six ramp parclo, laid out in my preferred configuration where the loops bypass the signals making them freeflow. In such a design you can have 6/8 turning movements freeflow, which beats any other weaveless single bridge design, though the 'straight on' movements on the surface street are still signalised. In this example two of the left turns have give ways rather than merges so might not be freeflow on an exacting definition; the presumably dominant turns (N to W and S to E) are freeflow via loops and left turn merges, while the other turns are deprioritised. The bridge here is very long, partly due to the skew, partly to match the wide central reservation of the 84 and to give more space for the signalised turns, but perhaps also with future proofing in mind - the 84 to the east has obvious space for an extra carriageway.
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Re: Australian Motorways

Post by Peter Freeman »

jackal wrote: Wed Aug 31, 2022 08:37
Peter Freeman wrote: Wed Aug 31, 2022 08:00
jackal wrote: Sun Apr 17, 2022 11:05 I have previously mentioned the extraordinarily low number of four way, full access freeflow interchanges in Australia ...
Anyway, this quirk is gradually being eroded ...
Here's another Australian 8/8, in Perth -
https://www.google.com.au/maps/@-31.964 ... 146,15.04z
Mentioned in my post of 19 April above.
No, the one you mentioned there is a 6/8, the interchange between Kwinana Fwy and Roe Hwy. My extra one is all movements, on Tonkin Hwy near to Perth Airport.
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Re: Australian Motorways

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Peter Freeman wrote: Wed Aug 31, 2022 08:46
jackal wrote: Wed Aug 31, 2022 08:37
Peter Freeman wrote: Wed Aug 31, 2022 08:00
Here's another Australian 8/8, in Perth -
https://www.google.com.au/maps/@-31.964 ... 146,15.04z
Mentioned in my post of 19 April above.
No, the one you mentioned there is a 6/8, the interchange between Kwinana Fwy and Roe Hwy. My extra one is all movements, on Tonkin Hwy near to Perth Airport.
For clarity here is the relevant bit of the quote.
jackal wrote: Mon Apr 18, 2022 09:37 My mystery 'offside stack' was probably an incorrect entry for the offside cloverstack outside Perth airport, which I did know about but had forgotten. This was constructed in late 2015. So that would be four four-way full freeflow interchanges in Aus, or three if we disqualify the Brisbane one with its signals.
Evidently I was talking about your '8/8' or 'all movements' interchange. (The point of confusion is just that I didn't link to that interchange, and later in the same post linked to some 6/8 interchanges, one of which happened to be in Perth.)

PS - I should have said 18 April, though you found the right post anyway.
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Re: Australian Motorways

Post by Peter Freeman »

Ah, my apologies - you had spotted it. I didn't look back far enough in your 18 April post to notice "outside Perth Airport".

Thinking about this further, the proportion of including all movements at 4-way freeflows in AU is curiously low. However, the proportion in UK is curiously high (as I think you alluded to earlier). In most of the AU incompletes, the main reason is that the missing movements are provided elsewhere nearby. For example, in Perth, not providing access to Murdoch Drive from Kwinana Freeway north is because the half-diamond just north of there provides it.

You would think this would occur often in the UK too. Perhaps it does, but, UK motorways being mainly rural and long-distance (AU's are urban and short), the resulting detours would be much longer. Another possibility is that the philosophy was "if we're going to build this interchange, we must do it properly (ie. completely)". Perfectionism, or 'gold-plating' as the expression on Sabre goes.
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Re: Australian Motorways

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Peter Freeman wrote: Wed Aug 31, 2022 10:49 Ah, my apologies - you had spotted it. I didn't look back far enough in your 18 April post to notice "outside Perth Airport".

Thinking about this further, the proportion of including all movements at 4-way freeflows in AU is curiously low. However, the proportion in UK is curiously high (as I think you alluded to earlier). In most of the AU incompletes, the main reason is that the missing movements are provided elsewhere nearby. For example, in Perth, not providing access to Murdoch Drive from Kwinana Freeway north is because the half-diamond just north of there provides it.

You would think this would occur often in the UK too. Perhaps it does, but, UK motorways being mainly rural and long-distance (AU's are urban and short), the resulting detours would be much longer. Another possibility is that the philosophy was "if we're going to build this interchange, we must do it properly (ie. completely)". Perfectionism, or 'gold-plating' as the expression on Sabre goes.
This is why we were all staggered when Ferrybridge (M62/A1(M)) was built as a partial movements free-flow interchange instead of a 3 level roundabout - the M18 being the missing movement for strategic traffic and the old A1 junction being for local movements.

When we want to design a proper junction we do an excellent job, but we're usually way too happy to make do with a 3 level roundabout because they're cheap and easy to add traffic lights to and then... uh, why not add an MSA as well.
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Re: Australian Motorways

Post by Peter Freeman »

Peter Freeman wrote: Fri Jun 05, 2020 03:41
crazyknightsfan wrote: Fri Jun 05, 2020 01:37 An influx of British engineers within Main Roads WA (HE equivalent) over the past decade means that there's starting to be a lot more of a British influence, the most obvious example being Main Roads' current obsession with roundabouts over traffic signals.
An unfortunate effect of this UK influence is Perth's recent implementation of two-bridge grade-separated roundabouts over motorways/DC's. This has also occurred elsewhere (eg. Hunter Expressway M15 in NSW), fortunately very limited. We're storing up a future problem, just like the UK has suffered. It's only a matter of time until the signals are added! It took me a long time to learn - now it seems that Australia's got to go through it.

Dubai (which I visit frequently as my daughter lives there) is similarly afflicted, their source also being UK traffic consultants.
Following up on the subject of British influence on contemporary Australian road design, I recently spent a few days in Perth, Western Australia, and, my partner being away visiting her family in Switzerland, I got in lots of road geekery.

I watched an online video about northward extension of Mitchell Freeway. Not the current extension, but the previous one. It was a good, informative video, featuring, as they sometimes do, interviews with engineers actually working on the project. All three were British ex-pats. One of them, a smart, well-spoken young man with a very recognisable UK accent, earnestly (condescendingly?) explained roundabouts, extolling roundabouts' advantages "over the signalised intersections that you've traditionally employed". The benefits were, correctly, the usual ones, with him emphasising "eliminating the need to stop". I was flabbergasted that he thought AU needed to hear that. We went through our roundabout honeymoon long ago, like the UK, but are now getting divorced. However, probably not in WA, where there is a concentration of British immigrants and little serious road congestion. The engineer concerned has probably not had time, since he immigrated, to visit Melbourne (after all, it's a long way), where we do have road congestion and are ripping up roundabouts as quickly as possible.

The backdrop to his interview was a tear-dropped dumbbell interchange (Hester Avenue, I think), obviously with no signals (yet).

As enumerated up-thread by CrazyKnightsFan, Perth now has eleven GSRs (grade-separated roundabouts, UK 2-bridge style). Six of them are in a cluster on Tonkin Highway, one on Joondalup Drive, one on Neaves Road, and three on Armadale Road. Two of the Armadale Road ones were quite controversial in the lead-up to opening. The press were referring to them as a 'duck-and-dive' interchange, and there was talk of 'spiralling' through the roundabouts. This all had me baffled until I visited last week. The two roundabouts in question are separated by only 450m. The D2 between them doesn't host any ramps, so you continue on C/D roads from the first roundabout to the next before seeing an on-ramp. In other words, they work as a pair. I finally realised the reason for the 'duck-n-dive' name: one end has the roundabout as its upper level, the other as its lower level. The ramp-less connecting road between them therefore has a gradient, and the C/D roads have a gradient the opposite way. This is emphasised by the proximity of retaining walls, which taper into or out of view as you proceed, giving a dynamic (still safe) feel as you drive.

The "spiralling": they're partially spirally painted (not visible on the animation, but can see on google earth) - and very explicitly, probably to allow for user unfamiliarity. One rather misinformed pre-opening commentator advised that you might have to go around twice to get into your correct exit lane ... !

Fortunately, Armadale Road is not a motorway, and elsewhere it has lower capacity intersections, so in this case its three GSR's will not become a problem. Ultimately, though, the Tonkin Hwy ones will.

I have to say that the duck-n-dive is superbly implemented, as recent Perth roads are. Excellent urban design, subways, NMU provision, technology, engineering, signage, aesthetics and artistry.

Location (satellite view is not up-to-date, but streetview and earth historical are) -
https://www.google.com.au/maps/@-32.127 ... 617,17.59z

Animated project overview video from 2020, prior to construction -
https://www.youtube.com/watch?v=Zqus8lCEIco
(added by an edit of this post in late 2023 - better late than never!)
Last edited by Peter Freeman on Thu Nov 16, 2023 23:29, edited 3 times in total.
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Re: Australian Motorways

Post by Bryn666 »

Being perfectly circular with tight entries along with the cycle provision actually seems they copied the Netherlands a bit there.

Certainly demonstrates far more ambition than the usual UK urban efforts which would have been a forest of at-grade signals.

The duck-n-dive element seems to be lifted from the A690 here https://goo.gl/maps/rDbvpKLd9K2FqyQp9
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Re: Australian Motorways

Post by Peter Freeman »

Bryn666 wrote: Wed Sep 07, 2022 19:10 Being perfectly circular with tight entries along with the cycle provision actually seems they copied the Netherlands a bit there.
This roundabout scale and shaping is the type that I like too. Negotiated at low safe speed, no pretensions of being a GSJ for massive traffic flows.

Bryn, you would love the cycling provision in Perth. There's a network called 'Principal Shared Path' (PSP). Each path is 2-3 metres wide, terracotta-coloured concrete, dashed centre-line resembling a vehicular road, and much (though not universal) grade separation from the road network. Every new road for many years, and many older roads, have them. The standard is consistently applied along major traffic routes, and penetrates right into suburbia. In Australian terms, it's rather similar to Canberra.
Certainly demonstrates far more ambition than the usual UK urban efforts which would have been a forest of at-grade signals.
Yes. But paradoxically, although I do like the end result in this case, simple signalised crossroads at these locations would have provided adequate capacity and much better value. Most junctions on this road, and other such roads in Perth performing the same role, don't have GSJs.
The duck-n-dive element seems to be lifted from the A690 here https://goo.gl/maps/rDbvpKLd9K2FqyQp9
Yes, similar in principle. Exactly the same distance between the roundabouts, and use of C/D roads. But those roundabouts are more UK standard, and are bigger, with more lanes. And despite similar proximity to housing, etc, the A690 vibe is minor rural HQDC rather than the built-up urban style of Armadale Road.

The UK road that this one most reminds me of is the A50 entering Stoke, westbound, between those vertical brick retaing walls for a mile or so. Not the same, but still that urban, bricky, comfortably-confined feeling.
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Re: Australian Motorways

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Peter Freeman wrote: Wed Sep 07, 2022 14:40I watched an online video about northward extension of Mitchell Freeway. Not the current extension, but the previous one. It was a good, informative video, featuring, as they sometimes do, interviews with engineers actually working on the project. All three were British ex-pats. One of them, a smart, well-spoken young man with a very recognisable UK accent, earnestly (condescendingly?) explained roundabouts, extolling roundabouts' advantages "over the signalised intersections that you've traditionally employed". They were, correctly, the usual benefits, with him emphasising "eliminating the need to stop". I was flabbergasted that he thought AU needed to hear that. We went through our roundabout honeymoon long ago, like the UK, but are now getting divorced. However, probably not in WA, where there is a concentration of British immigrants and little serious road congestion. The engineer concerned has probably not had time, since he immigrated, to visit Melbourne (after all, it's a long way), where we do have road congestion and are ripping up roundabouts as quickly as possible.

The backdrop to his interview was a tear-dropped dumbbell interchange (Hester Avenue, I think), obviously with no signals (yet).
Interestingly, Hester Avenue roundabout was opened with metering signals on the freeway approach which have since been removed.
https://goo.gl/maps/hwrRWAdNBagYfT6z6
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Re: Australian Motorways

Post by Peter Freeman »

^ That is interesting Sam. I suppose they thought, with that interchange being temporarily the terminus of the freeway, that the freeway traffic would overwhelm and frustrate the local approaches. But it didn't happen ...?

I have looked there on Google Maps, to check for congestion. There is queueing at the evening peak - significant queueing, by Perth standards. I suppose that's inevitable at a freeway temporary terminus (which this exit still is). It suggests that the approach metering should have gone elsewhere, to let the northbound evening queue escape. Best to leave it for now though, as this exit will be de-stressed by the impending Mitchell extension.

Roundabout approach metering, as opposed to the signalisation style extensively used in UK, has been installed often in Australia, although only one of Melbourne's several examples is still in place. It generates gaps in the dominant flow. I know at least one is used in Perth, at the Mounts Bay Road roundabout, and I have observed it to be effective.
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