jackal wrote: ↑
Wed Sep 08, 2021 08:56
Once upon a time I used to say long slip roads weren't true C/D lanes, as C/D lanes are supposed to split local and express traffic and to do that they have the feature of connections to/from the mainline at both ends. Here the split is between different types of strategic traffic (M4/M25) so there is no need for outercarriageways to link to the M25 mainline at both ends. Other examples of 'long slip roads' are M8/M74/M77 and the forthcoming M2/A2/A289/LTC section; examples of 'true C/D lanes' are M60 J6 to J8 and M20 J5 to J6. I've long accepted that you have to call both types C/D lanes as everyone else does, and they're clearly close relatives.
Yes, close relatives, in the extended family of 'multiple carriageways', and there are some hybrids. However, I think the distinction is important.
Long slips most often occur when a suitable (eg. more spacious) area must be reached before a braid can fit in, or to avoid construction impediments, or sometimes to avoid having to build an extra structure ... etc.
Genuine C/Ds mainly occur in order to insulate express lanes from minor junctions, but sometimes to provide a lower-speed, and safer, zone where weaving within a short space between junctions is tolerated. It's a lower cost option than the preferable braid. For example, J15/16 on the M5 at Almondsbury. The C/Ds within some clover-leaf GSJs are a special and trivial case, but the same logic.
Our Melbourne Australia freeway network has only one pure (ie. non-braided) C/D implementation (M1 Westgate Tunnel Approach, under upgrade to Q3), plus three that are actually to facilitate braiding but almost incidentally do
re-join (on M1 and M2). However, we have pure braids, some reached by long, non-rejoining slips.
[ I believe the main reason for our pure C/D is so that Transurban can toll (HGV's only) the outer carriageways that lead primarily to its tunnel: vehicles on the inner carriageways that lead to the bridge will not be tolled. Otherwise it could simply be D5, or even D6 as there is space. The equivalent M1 on the other side of the city is D5, without any weaving problems. ]
I'm not a fan of C/Ds. Often they waste space and capacity. If there's sufficient width to build a Q3 with 6 lanes total each way, then generally there's width to build a D7. This creates an 'auxiliary lane' solution to weaving. Right lanes are insulated from left-lane weaving disruption simply by their distance from it, rather than by a barrier. There's nothing quite like extra space.
(OT digression over)