nanog mailing list archives
Re: OT: Traffic Light Control (was Re: First real-world SCADA attack in US)
From: Jay Hennigan <jay () west net>
Date: Wed, 23 Nov 2011 15:16:48 -0800
On 11/23/11 2:52 PM, Jay Ashworth wrote:
Well, sure: what's the *incidence* of conflicting greens? I wasn't suggesting that the incidence of accidents would be any different between conflicting greens and other types of failures (though my intuition is that it would be higher), but that's swamped by how often the condition actually occurs, which, appears to require someone physically running a truck into the control box, or a chain of 5 or 6 failures in cascade to occur, based on other postings on this thread.
Real-world scenario that actually happened: There is an intersection where a majority of the E/W traffic makes left turns to N/S. The signal there has three phases. N/S solid green, East solid green with left arrow (protected left turn) and West with solid green and left arrow. East and West are never green simultaneously, this would be a conflict due to the full phase protected left turns. At some time unknown the controller was replaced and a stock N/S vs. E/W conflict monitor wound up in the box. Nobody owned up to this. (Human error, sloppy procedure, and lack of audit trail.) The programming of the controller was OK, however and the intersection ran just fine. Time passed, probably months. Something glitched the controller and it crashed. This also put the intersection into four-way flash. A somewhat inexperienced technician arrived on scene rebooted the controller and it went back to factory defaults which are N/S vs. E/W. Had the conflict monitor (a circuit board with a diode array, hardware - not software) been correctly programmed for that intersection, it would have kicked back to flash. No problem. But it wasn't. And because the left turn arrows were hard-wired in the signal heads to the same wire as the solid green phase, there was a conflict. Fortunately the technician heard the blaring horns and witnessed a couple of near-misses before an accident occurred. He put the intersection back on flash, dug out the print for the conflict monitor and programming, called for help, and got it fixed. Normally sane defaults in a non-standard configuration, sloppy procedures, and human error coupled with a failure.
From a practical standpoint it is difficult for one person to observe
more than one or possibly two phases, especially from the location of the controller which is typically placed a few feet away from the corner so that it gets run over less frequently.
As such, I'd say that the probability of a conflicting green occurring and causing an injury accident is pretty low even with (relatively) modern digital signal controllers.Yup, it does appear that's true.
But it happens. -- Jay Hennigan - CCIE #7880 - Network Engineering - jay () impulse net Impulse Internet Service - http://www.impulse.net/ Your local telephone and internet company - 805 884-6323 - WB6RDV
Current thread:
- OT: Traffic Light Control (was Re: First real-world SCADA attack in US) Jay Ashworth (Nov 22)
- Re: OT: Traffic Light Control (was Re: First real-world SCADA attack in US) Brett Frankenberger (Nov 22)
- Re: OT: Traffic Light Control (was Re: First real-world SCADA attack in US) Jay Ashworth (Nov 22)
- Re: OT: Traffic Light Control (was Re: First real-world SCADA attack in US) Brett Frankenberger (Nov 22)
- Re: OT: Traffic Light Control (was Re: First real-world SCADA attack in US) Owen DeLong (Nov 22)
- Re: OT: Traffic Light Control (was Re: First real-world SCADA attack in US) Jay Ashworth (Nov 23)
- Re: OT: Traffic Light Control (was Re: First real-world SCADA attack in US) Jay Hennigan (Nov 23)
- Re: OT: Traffic Light Control (was Re: First real-world SCADA attack in US) Jay Ashworth (Nov 23)
- Re: OT: Traffic Light Control (was Re: First real-world SCADA attack in US) Jay Hennigan (Nov 23)
- Re: OT: Traffic Light Control (was Re: First real-world SCADA attack in US) Jay Ashworth (Nov 22)
- Re: OT: Traffic Light Control (was Re: First real-world SCADA attack in US) Brett Frankenberger (Nov 22)
- Re: OT: Traffic Light Control (was Re: First real-world SCADA attack in US) Bryan Fields (Nov 23)
- Re: OT: Traffic Light Control (was Re: First real-world SCADA attack in US) Valdis . Kletnieks (Nov 23)
- Re: OT: Traffic Light Control (was Re: First real-world SCADA attack in US) Mark Radabaugh (Nov 23)
- Re: OT: Traffic Light Control (was Re: First real-world SCADA attack in US) Robert E. Seastrom (Nov 23)
- Re: OT: Traffic Light Control (was Re: First real-world SCADA attack in US) Brett Frankenberger (Nov 23)
- Re: OT: Traffic Light Control (was Re: First real-world SCADA attack in US) Thomas Maufer (Nov 23)