The documentation is now in pdf format:
www.cyberpictures.net/wlt.pdf amd so those that were nervious about
downloading a MS Word document should be happier with this format. I
appreciate the efforts made by Colin Rosenstiel to bring this happy
state of affairs about.

Meanwhile with the help of my friend I would like to give a response
to Dave Arquati at Imperial College. Colin Mackenzie points out more
fallacies and doesn't know about kerb guidance. He might have a point
about decelleration though. So here is what we feel is an appropriate
response. If you have not the time or inclination to read on further,
now is the time to move onto other topics in this news group.

It is felt that the reduced speed approach problem quoted is more
theoretical than practical in reality. Clearly to get real answers to
the traffic flow difficulties associated with both the 3 minute 40
metre tram option with banned turns and the 2 minute
25 metre trolleybus option with permitted turns requires some very
complex traffic flow mathematics and is best done with suitable
computer simulations. If TfL were really interested in checking out
various options in an honest and open manner, such simulations could
be easily arranged using one of a number of reputable suppliers.

Having no access to such simulations, nor the funds to purchase same,
I give some approximations of what might occur using much simpler
approximated mathematical models.

If we assume that the road traffic flow is 30 m.p.h. (around 12
metres/second) a tram will decelerate to rest at maximum service brake
force in around eight seconds. If we assume that the trolleybus has a
mean deceleration of one third less to gain the Kassel kerb (that
brings the deceleration rate down to around 1 metre per second squared
and is probably a much greater reduction than required in reality),
then the trolleybus will take 12 seconds. At first sight this appears
to give the tram an advantage in time of four seconds per stop but it
has to be remembered that the trolleybus has a very great advantage in
acceleration and although very high maximum accelerative rates are
quoted for modern trams, these are actually very theoretical and in
practice only apply on the level or downhill and with dry rails. If
there are any adverse adhesion conditions, the electronic control
systems compensate to prevent slipping and acceleration is actually
much less in practice than the theoretical maximum quoted. The
trolleybus with its rubber tyres on the road does not suffer the same
loss of traction (this of course is the reason for the rubber tyred
metros in Paris which have much higher accelerative rates in practice
than the conventional steel wheeled variety). So in practice the
trolleybus is not actually going to lose any time.

Now in respect of road space occupied and thus clearance behind the
vehicle the following applies. (For any mathematicians, the figures
that follow are based on linear extrapolations rather than using
integral calculus as I do not have the precise braking curves and the
difference will be too small to affect the overall results). The
trolleybus in its 12 second deceleration will traverse about 72 metres
whilst in the same 12 seconds the tram if able to go at full speed for
the first four seconds before deceleration will traverse 96 metres in
the same time. The difference is 24 metres less the 15 metre disparity
in length of vehicle and so equals 9 metres (nearly two car lengths).
This appears to suggest that the tram has an advantage (albeit slight)
but of course these figures are only true if the traffic is moving at
30 m.p.h. which would suggest clear road conditions with minimal
disruption.

If we assume a greater level of traffic such that the tram or
trolleybus can only approach at 20 m.p.h. (8 metres per second), then
the figures are very different. The tram now takes 5.5 seconds to
stop. Because of the lower speed approach, the trolleybus may now be
able to decelerate at the same rate as the tram, but even assuming the
same two thirds ratio as previously, the trolleybus will now take
around 8 seconds to stop. The difference now is that the tram has gone
about 42 metres whilst the trolleybus has gone about 32 metres. The
difference is 10 metres which is less than the 15 metre length
difference. This implies that the trolleybus now clears the road
behind quicker because of its lesser length and despite any lesser
decelerative rate.

So the conclusion appears to be that the tram is only advantageous in
clearing the road behind when entering a stop if the road is clear. If
the Uxbridge Road were likely to be clear, then of course there would
be much less debate about the whole scheme. The more congested the
road and thus the lower the mean speeds along the more congested
sections, the more advantage the shorter trolleybus has in this
respect. The trolleybus also has the capacity to manoeuvre in two
dimensions around congestion and/or stationary streams of traffic
waiting to turn. The tram has a one dimensional fixed path on its
track.

Combining the two, the only reasonable conclusion must be that the
trolleybus has the advantage in terms of congestion effects on other
traffic, when the traffic is already congested. The tram would only
have a theoretical advantage when there was little congestion and the
traffic was free flowing and thus therefore when such theoretical
advantage was of no practical value.

OK you can all leave the classroom now. Bye.

David Bradley