[sustran] Re: Bus and rail (fwd)

Kerry Wood kerry.wood at paradise.net.nz
Fri Mar 1 12:04:22 JST 2002


More issues to consider

1)    Capacity

Some of the busway capacities quoted are greater than light rail can manage.
That's great if you can do it, but it might be a good idea to think about why.

Two hundred passengers in each unit of a triple LRV at 90 seconds headway = 24
000 pass/h. It  may look OK on paper, but the units need to be no longer than a
city block and the cross traffic needs to get a look-in.

More practically, a LRV can carry 400 passengers and clear a junction from a
standing start in 12 seconds. Call it 18 seconds of junction time, and allow for
both directions and say 2 minutes headway. On this basis light rail is using 30%
of total junction time (both directions), for a capacity of
12 000 pass/hr. Near-absolute priority for light rail is possible.

Eight 50-passenger buses will need a lot more than 18 seconds, even if they can
manage a neat convoy (I am still thinking how to work out their junction time).
Random arrivals in say 80-passenger bendy-buses will need 150 buses an hour, or
300 buses/hr for both directions. Allow say 12 seconds of junction time each and
the buses will need just 100% of junction time, or a little bit less allowing
for queuing and simultaneous arrivals. Near-absolute priority is not an option:
it is essential.

On-street light rail is unlikely to be dearer than buses on an overpass. And
very unlikely to be noisier. Make your own assumptions and plug in your own
numbers, but do the sums.

2)    Alignment width

Bus lane width is much the same as light rail for guided buses, greater for
manual guidance, or much greater if stopping bays or overtaking lanes are
needed.

A paper by G Gardner ("A study of high capacity busways in developing
countries," Proc Inst Civ Engrs, Transp 95, 8/1992) shows a Sao Paulo layout as
3 lanes wide, plus the stop platform: Allow say 2.5 m for the platform, 3.5 m
for the stopping lane and 2 x 4.0 m for through lanes (extra width to get bendy
buses through the S-bend between offset platforms for the two directions) and
the minimum width needed is about 14 m. Gardner shows another layout, from Belo
Horizonte, which does not use offsetting: Say 2 x 2.5 m platforms, 2 x 4.0 m
laybys and 2 x 3.5 m through lanes, or about 20 m.

The equivalent width for light rail might be 2.5 m + 2 x 3.5 m, or 9.5 m for
offset stop platforms, or 12 m without offsetting.

3)    Continuity

Obviously, light rail tracks have to be continuous. Bus routes have to be
continuous too, but it is less obvious that the priority needs to be as
continuous as for light rail. Political pressure to give up on continuity might
be very hard to resist. This may not matter too much in the suburbs, but in the
city centre it is a very different matter. The city-centre continuity of the
Ottawa busways was never more than an underground pipe-dream, and now Ottawa is
considering light rail.

4)    Journey time

This is the objective, at least in richer cities. If public transport journey
time effectively controls the speed of traffic (Martin Mogridge's theory: I have
found some evidence for it in the ISTP data) this this is what matters. It is
probably NOT what matters in all cities, but it seems to be important in a lot
of cities, large and small.

And fast, reliable journeys need good interchanges, which in turn need good
timekeeping, which needs a delay-free route, which needs high priority on most
or all junctions.


--
Kerry Wood
Sustainable Transport Consulting Engineer





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