On Sun, 7 Sep 2008 14:28:56 -0700 (PDT) someone who may be Mizter T
wrote this:-

Sorry, I didn't make myself clear. In Croydon, as is the case
elsewhere with other tram systems, the highway traffic control signals
are linked in to the tram's signalling system so as to enable trams to
negotiate a variety of road junctions

The tramway signalling system [1] makes a request to the traffic
light controller. The request informs the traffic light controller
that a tram is approaching or waiting depending on circumstances.
How the traffic light controller responds to this request depends on
the junction and the circumstances at the time. At one extreme it
will respond immediately and change the other lights before giving a
proceed signal to the tram driver. At the other extreme it will note
the tram request and fit it in when a suitable opportunity arises.

The priority of a tram request may be increased if the tram comes to
a stand at the junction. It all depends on the junction and how that
is linked to other (road) junctions.

Many road junctions may be linked and the road bods may not give
trams absolute priority. However, trams are carrying many people in
a small amount of road space and thus can be given a high priority
than other forms of traffic.

[1] generally just detector loops. An advance detector is placed
some way in the rear of the signals. A stop detector is placed at
the stop mark. A cancel detector is placed after the lights, if the
proceed signal has not already been replaced by the white dot then
this detector will initiate this. One detector can perform more than
one function, for example a cancel detector can also function as the
advance detector for signals further along the tramway.


--
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