Re: OT: Traffic Light Control

[Robert Bonomi <bonomi () mail r-bonomi com>]

On Tue, Nov 22, 2011 at 02:26:34PM -0500, Jay Ashworth wrote:
> > Some other things to consider.
> >
> > Relays are more likely to fail. Yes, the relay architecture was
> > carefully designed such that the most failures would not result in
> > conflicting greens, 
>
> My understanding was that it was completely impossible.  You could 
> fail dark, but you *could not* fail crossing-green.

Just to put one point to rest.

I, personally, have witnessed traffic lights showing 'green both directions'.
*TWICE*.  One was in the mid-1960s, with what was undoubtedly relay-based 
control logic; the second was in the late 1990s, *probably* with solid-state
'management' controls , but I don't know for certain.  (The 'relatively
recent' unit's I've seen the insides of have solid-state logic driving final
'output' relays that provide power to the actual signal head.)

In the first case, the pedestal-mounted control unit had been subjected to
excessive impact forces, and some of the 'output' wires had shorted together.