Traffic Fiasco?

[TubaTodd]

I saw this article on Slashdot.org and immediately thought of Rick Denney....who I believe works in the world of traffic.

http://tech.slashdot.org/story/09/11/05 ... -MD-County

Your comments kind sir?

[windshieldbug]

I'm willing to bet that it has something to do withthem using a 50 year-old "computer" system...

[Rick Denney]

I'm willing to bet that it has something to do withthem using a 50 year-old "computer" system...

No, they use an Eagle Comtrac system which is not 50 years old. It's only 30 years old. It does use a real computer, though. I think in their case they are using a Concurrent (another version of the Perkin-Elmer that was used for many, many traffic signal systems in those days). Old stuff, to be sure, but it really is digital.

Now, the Phase Rotation system that I retired in San Antonio would now be 50 years old, and it used vacuum-tube hardware and an analog system that communicated signals over the 120VAC service lines by altering the phase relationship. Pretty slick stuff for 1957.

Replacing their system is easier than they think. They need to work from the signal controllers up and not from the computer down. All modern signal controllers sold in the U.S. have a background coordination capability known as time-based coordination. (British controllers have that, too--it's called Cableless Linking.) The controller gets its coordination signal timing from a table stored in the local controller. It sequences the movements based on those timings, and lines up with other intersections by deriving the timings from a real-time clock. As long as the real-time clocks are in synch, the system stays properly coordination. This is the backup mode of operation for all modern signal systems.

Had Montgomery County installed time-based-coordination signal timings in their controllers and then configured their remote system interface units to revert to that operation in case of loss of signal, the fiasco yesterday would probably not have happened. They could even now program their controllers for TBC, install a new computer management system, and then transition from the old system to the new one communications circuit at a time.

I put 700 intersections under computer control in San Antonio in about a year, making use of existing communications cable for at least 400 of those intersections. And it will stay in coordination even if the whole comm network and central computer goes away. That was in about 1990.

Their Comtrac system depends on second-by-second communication from the central computer to the signal controller at the intersection, with the central computer directing each change in the signals (except clearance intervals, which are timed locally). Most current systems let the controllers do their own signal timing, and use the computer to manage the controller databases and provide real-time monitoring.

Uh, yes, I know a thing or two about this subject.

Rick "noting that Montgomery County is usually out front of everyone else in these parts with traffic system deployment" Denney