On Sun, 07 Sep 2008 22:26:26 +0100 someone who may be Graham Murray
wrote this:-

On the other hand, where a tram is running along a road, is there any
need for it to have separate signals? Would it not suffice for it to
have to obey the same red/amber/green traffic lights as other traffic?

Yes, which is why separate signals are provided.

There are many reasons why tram drivers may be given separate
signals, for example if a tram is to start off before other traffic
at a junction a separate signal is needed. The "amber" period for a
tram is a few seconds longer than for motor traffic. It is also
considered less confusing for operators of other forms of traffic.

Having decided to provide separate signals for tram drivers the next
question was what form these signals would take. The two contenders
were tram stencils on coloured lights (just as there are bike (and
possibly bus [1]) stencils in places) or white lights. In essence
the white light system was chosen as there is less chance of
confusion with signals for other forms of traffic.

White light signals could be provided by standard traffic light
units with clear lenses and the appropriate stencil. However, the
"searchlight" unit was chosen as it is smaller than a three light
traffic light unit (being fitted to the top of a pedestrian crossing
pole is a lot neater than having a second traffic light unit beside
the one for other traffic) and can provide a greater variety of
indications.

Given the limitations of operating on the roads I think the system
would be hard to improve on.



[1] I have a very vague recollection of having seen bus stencils
somewhere in the UK. Bike stencils are provided in places though,
unlike some other places, there is no stencil on the red lens.


--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54