jackal wrote: ↑Sun Mar 03, 2024 21:37
Addendum B includes a rejected alternative with C/D lanes (yes, proper ones, with their own grade separation):
R300 alternative - Copy.JPG
For the usual valid reason I suppose - De Bron, Legato and Williams
are all fairly close together. But rejected anyway, of which I approve: too extravagant!
My view (which I've expressed previously, and I know is not shared) about C/D lanes for weaving mitigation is that they're often not the best (ie. most cost-effective) solution. They introduce unnecessary build complexity, navigation confusion, signage clutter, and they waste space. I believe equal mitigation can be achieved simply by generous lane provision: instead of 2-3-3-2, build D5.
The alternative, where exit volumes are high enough, is an auxilliary lane between junctions - every junction is a lane-drop/lane-gain. The northern stretch of Melbourne's M80 is like this. Also, as you stated, R300 will use this alternative: D4 between De Bron and Legato.
Surprisingly, in Melbourne we have few C/Ds, even though a casual glance reveals several areas of >2 carriageways. Most of these are actually early-diverging or late-merging simple ramps, for reasons of fit or for braiding. Even braiding is not frequent.
Melbourne's only long (7km) stretch of genuine C/D has just been commissioned on the now-Q3 Westgate Freeway: between its M80 interchange and the Westgate Bridge it has D3 inner express lanes. When you take shoulders and hard-strips into account, this whole length at similar (or lower) cost could have been D7. I'm certain that there will be confused drivers ending up on the bridge when they really wanted the tunnel.
To get to the bottom of this arrangement, tolling, not just weaving, must be taken into account. The tunnel will be tolled, but the widened Westgate Freeway will remain free, since it always has been. However,
for trucks only, the
outer carriageways only WILL be tolled. The C/D arrangement facilitates this distinction. The whole project was initiated by Transurban, of course ...
A shorter stretch of genuine C/D is at the fork of M2 and M79. Outbound, a C/D diverges left to deal with the Bulla Road and English Road interchanges. It re-joins M79 after 1.5km, and M2 after 2.5km. Inbound, C/Ds diverge from M2 and M79 to deal with Bulla Road and Bell Street, rejoining M2 5km downstream.
I thought even the proposed D3M was extravagant, given how sparsely populated the area north of an extended R300 would be. But apparently there's a proposal for a city of 800,000 people, called Milkwood City, north of where the R300 would meet the N7. That'd do it.
Fascinating. I just read-up on Milkwood. However, I won't hold my breath to see that.