Tony is right in the particular circumstance of shared track with heavy rail (other than when the sharing is on a time-share basis).

Ditto when there are sight line issues. BUT, suppose there is a sharp bend in a route with a sight line issue - the correct procedure is not to install an expensive block signalling system for the whole line, or even for that particular section. The correct approach is to impose a speed limit, so that any tram approaching the corner will be at a speed such that it can stop before hitting any obstruction.

Then there is the general question of visibility. If your system is subject to fog - how often is there good fog in Melbourne - you need to consider speed reductions in fog. The following table is based on a table in:

http://www.wolstanton.org.uk/public_html/web_weather/downloads_weather/BeaufortCode.pdf

with the Visibility Code replaced by columns for speed in m/s and km/h.

Object
Standard Distance
Visibility Description
Maximum Speed ms-1
Maximum Speed km /h

None
<20m
Dense Fog
Walking
5

A
>20m
Dense Fog
8.9
32

B
>40m
Thick Fog
12.6
45

C
>100m
Thick Fog
20
72

D
>200m
Fog
28
100

E
>400m
Moderate Fog
na
na

F
>1000m
Very Poor
na
na

From the formula for constant deceleration,

V2 = U2 + 2as, where V is the final speed = 0, U is the initial speed in ms-1, a is acceleration - assumed to be -2 ms-2 and s is the visibility distance in metres.

Hence U = 2s-2.

To be on the safe side one should use perhaps the normal deceleration rate rather than a rather higher, but not full emergency rate, and should allow normal reaction time (0.7s) to adjust the figures downwards. As an added precaution, perhaps a switch for use in fog only which will turn on the rear lights brighter than normal and cause them to flash - more easily detected than a steady light.

And if you have a subway where it may not be easy to estimate the distance and speed of the tram ahead, there is always the 'proximity' alarm, used by AA gunners in WW2, and every since, but in their case to detonate an explosive if the distance was reduced sufficiently. Similar in effect to the ground proximity alarm on aircraft.

All much simpler, cheaper and more effective than a signalling system.

Finally, if you have to have a subway and you MUST have a junction in it, you will need signalling as by definition the merging routes will be very line of sight limited.

Regards

Dudley Horscroft
----- Original Message -----
From: Tony Prescott
To:TramsDownUnder@...
Sent: Thursday, June 16, 2011 9:17 PM
Subject: [TramsDownUnder] Tram Regulation [was Re: High floor and rotating trucks [Was: Tram seating and access]

--- InTramsDownUnder@..., "Noel Reed" <noelreed10@...> wrote:
>
> Hello Tony,
>
>
>
> In what way does the SLR signalling system 'thwart operational efficiency' ?
>
> When we introduce new tramways on dedicated right of way, should we take no
> notice of the safety of passengers by ignoring technical improvements to
> rail systems ?
>

Because it's a road transport system interacting with other road transport, not a stand-alone rail transport system. Nobody would suggest ordering buses into signalled block working. As you say, road traffic controls (lights and signs) are the "signalling system" for all road users. Apart from that the driver needs the flexibility to use their judgement to adjust their driving to changing circumstances in the road environment - even if in reservation.

And apart from all that it's another up-front expense, another maintenance burden - and another thing to go wrong.

The only role for signalled safeworking for trams I can see are 80 km/h + reservation running where there are sightline issues and where the rail is shared with other external rail uses (heavy rail).

cheers
Tony P

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