On Tue, Nov 22, 2011 at 02:26:34PM -0500, Jay Ashworth wrote:
Yes, but the complexity of a computerized controller is 3-6 orders of magnitude higher, *and none of it is visible*
You can't see the electrons in the relays either.
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.
If properly wired, maybe. But probably not. I'd have to see the architecture, but, for example, is there any risk of a power surge at the wrong time welding the green contacts togethor and resulting in a permanent green in one direction that doesn't lock the other direction out? (Maybe not. But I'm confident that some failure could be contrived with a detailed explanation of a real system.)
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,
By 2 or 3 orders of magnitude, usually; the second thing they teach you in driver ed is "a dark traffic signal is a 4-way stop".
Of course, not everyone follows the rules. People learn "red means stop" well before driver ed, but sometimes they don't stop, even at a red. Traffic Signal Out cases are often a mess, because you have a relatively complicated or busy intersection, and a collection of drivers only 95% of which (for example) actually know how to handle the case, and even many of those 95% are very tentative because they know that not all the other drivers know the rules. The upside is that most of the collisions that result are low speed.
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.
The underlying issue is trust, as it so often is. People assume (for very good reason) that crossing greens is completely impossible. The cost of a crossing-greens accident is *much* higher than might be imagined; think "new Coke".
New Coke was imposed on all coke drinkers, though. A better analogy is airline plane crashes. They are exceedingly rare, but when they happen, almost everyone knows about them. Yet people still fly. Even immediately after the crash. (And a modern airplane is orders of magnitude more complicated than a solid state conflict monitor.) Conflicting greens are also exceedingly rare, and it's not nationwide or worldwide news when they occur. If conflicting greens start occuring routinely, yeah, people are going to lose confidence in the system. But we could likely withstand a couple orders of magnitude increase in the number of green-on-green incidents without any meaningful reduction in confidence in traffic signals. Still, obviously, the point isn't to keep increasing the frequency of conflicting green incidents until people start to lose confidence. The point is that there's no evidence of any meaningful increase in risk with electronic controllers.
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.
Sure. But we have no numbers on either side.
Yeah, and I looked. There's nothing I could find. But ... I'd be shocked to find evidence of a statistically higher risk of conflicting greens in electronic conflict-monitor implementations over relay-based systems of comparable intersection complexity. Vital electronics is a well-established industry. We all work in a bug-of-the-week industry, where we demand more speed, more features, and so on, and accept quite a bit of risk associated with that. Even the careful networks that nominally place a high value on stability don't have a reliability comparable to a traffic signal or an airplane. But that doesn't mean reliable electronic systems can't be built. Just that you have to prioritize that over other things if that's what you want. And that's what the vital electronics industry does. -- Brett