Re: It's still academic

["David & Jan Winter" <winterd@icenet.com.au>]

Eddie Oliver wrote in message <36D37C7F.4132@efs.mq.edu.au>...
>MattyQ wrote:
>
>> >What about Yellow over Yellow:
>> >
>> >NSW: - diverge, next sig at caution?
>> >VIC: - 3 pos repeater, nest sig at stop
>> >QLD: - next sig at Single yellow, dunno the proper name of the aspect
>>
>> YES !!!! We have a winner !!!
>
>No you don't - not quite. NSW yellow over yellow does NOT mean diverge,
>next signal at caution. It just means diverge, next signal AT LEAST AT
>caution or showing yet another turnout indication. Yellow over yellow is
>the most unrestrictive indication obtainable for a turnout, so even if
>the next signal is at full clear, you can't get anything "clearer" than
>yellow over yellow.
>
>Various reasons have been suggested for the lack of yellow over green as
>the "natural" aspect in the last-mentioned situation, but the people who
>would definitively know are long since unavailable. The most popular
>theories are (a) they didn't want to create an inversion situation where
>someone might thoughtlessly interpret yellow over green as meaning green
>over yellow and (b) they didn't want to downgrade the immediate impact
>of the top yellow by having a green on view as well.

FAIL SAFETY would be a consideration: if a green/yellow indication meant a
more restrictive condition than green alone, and the yellow failed, an
unsafe situation could emerge. Of course, driver route knowledge SHOULD be
such that the absence of a 2nd aspect would cause the driver to assume a
fault and move to the most cautious approach ... BUT ...

DW

>