James wrote:

I have long been a fan of such an idea. This is borne out of experience
of express routes running in the old Met county areas like Tyne and Wear
and West Yorkshire where a multi centred conurbation can support such
services. The other key example which works well is Hong Kong which has
a hierarchical bus service network.

From my experience of it in West Yorkshire, it doesn't work very well.
The Express X84 runs (with a minor deviation) out of Leeds as far as
Lawnswood with the local 1 and 95. It is only express in the sense
that it picks up only outbound and sets down only inbound within the
Green Zone. When I have ridden it, the only place where it passed a 1
or 95 is inbound where the 1 and 95 diverge off the A660 to stay on
Woodhouse Lane and serve the University. Most of the benefit of this
express run was in the driver's sadistic enjoyment of slamming the
doors shut in people's faces in Headingley. I didn't even save any
time as I had to walk down from the Headrow to the Railway Station,
which the 1 would have dropped me near.

From my experience of it in South Australia, it does work very well but
I doubt it would be at all well suited to London.

Here in Adelaide there are three different types of express buses. Those
closest to as you've described are numbered with an F suffix. They're
good for passengers from outer suburbs who benefit from significantly
reduced journey times, but unlike the non stop buses (X suffix) they do
take these passengers to destinations short of the City. And although
they don't stop to pick up passengers going into the City, some drivers
will let passengers on if they've stopped already (as long as there
aren't too many, as the boarding process is slower than that of London).

The third kind of express buses are those which only serve a few of the
stops along the route. These routes are numbered with a T prifix,
although not all of them are based on stopping routes, and those that
are avoid taking the deviations that the stopping route takes.

I don't think any of these would work so well in London. The main
obstacle is the road congestion in Inner London. Bus lanes have gone
some way to alleviating this, but very few of them are wide enough to
permit overtaking (and places wide enough to overtake are are often at
the more important stops), so the benefits would be limited. Also the
dominance of the City (or even of Zone 1) is much lower in London, so F
type buses would be stopping a lot more than they do here. London also
has a much more comprehensive rail system, so there are fewer places
where express buses would be more convenient than trains.

London used to have quite a lot of express bus routes, but rail
improvements led to the abandonment of the D1, X53 and X130. There are
some quite successful commuter routes from the Home Counties, and
Greater London still retains a few express routes such as the 726
(albeit in truncated form) and the X68 (peak hours only IIRC). However,
there are few fans of express buses now. There was an exception a few
years ago - one of the mayoral candidates announced a bold and
imaginative plan for a large network of radial and orbital express bus
routes. Unfortunately (or perhaps fortunately, as the plan had some
major flaws) it was Jeffrey Archer.

So what should London do instead? I supplied the answer a few weeks ago:
run untimetabled (but frequent) routemasters in addition to the regular
service! With drivers instructed to go as fast as they safely and
comfortably can, the RMs would regain their reputation for being fast,
and their lack of accessibility would not be a problem as the entire
route would still be serviced by low floor buses.