On Tue, Nov 22, 2011 at 11:16:54AM -0500, Jay Ashworth wrote:
----- Original Message -----
From: "Owen DeLong" <owen@delong.com>
As in all cases, additional flexibility results in additional ability to make mistakes. Simple mechanical lockouts do not scale to the modern world. The benefits of these additional capabilities far outweigh the perceived risks of programming errors.
Relay logic has the potential for programming (i.e. wiring) errors also. It's not fair to compare "conflict monitor" to "properly programmed relay logic". We either have to include the risk of programming failures (which means "improper wiring" in the case of relay logic) in both cases, or exclude programming failures in both cases.
The perceived risk in this case is "multiple high-speed traffic fatalities".
Some of the benefits of the newer systems are safety related also.
I believe we rank that pretty high; it's entirely possible that a traffic light controller is the most potentially dangerous artifact (in terms of number of possible deaths) that the average citizen interacts with on a daily basis.
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, but that's not the only risk. When the traffic signal is failing, even if it's failing with dark or red in every direction, the intersection becomes more dangerous. Not as dangerous as conflicting greens, but more dangerous than a properly operating intersection. If we can eliminate 1000 failures without conflicting greens, at the cost of one failure with a conflicting green, it might be a net win in terms of safety. Modern intersections are often considerably more complicated than a two phase "allow N/S, then allow E/W, then repeat" system. Wiring relays to completley avoid conflict in that case is very complex, and, therefore, more error prone. Even if a properly configured relay solution is more reliable than a properly configured solid-state conflict-monitor solution, if the relay solution is more likely to be misconfigured, then there's not necessarily a net win. Cost is an object. If implementing a solid state controller is less expensive (on CapEx and OpEx basis) than a relay-based controller, then it might be possible to implement traffic signals at four previously uncontrolled intersections, instead of just three. That's a pretty big safety win. And, yes, convenience is also an objective. Most people wouldn't want to live in a city where the throughput benefit of modern traffic signalling weren't available, even if they have to accept a very, very small increase in risk. -- Brett