The truth about traffic

By Dead Eye Dick

/keep you and me out jail stuff/ The content of this document is purely for educational purposes. To satisfy those who would be curious. Improper use of the information in this document, while it would not enable a person to cause a traffic accident, could effectively alter traffic flow in such a way to prevent emergency vehicles from reaching their destination in a most efficient manner. In other words, yeah this stuff is interesting but don't use it to mess with the traffic system!
/keep you and me out of jail stuff/

I'm sure you all have seen them. You know, those gray, metal cabinets that stand on every street corner? You may have also noticed that not every street corner has one. What seems to be the deciding factor is, that for every street corner that has one of these horribly unobtrusive gray cabinets, there is also a traffic signal. Indeed, inside the gray cabinet resides the equipment whose sole purpose in life is to turn on and off the traffic lights in such an order that will facilitate (city engineers hope) the efficient flow of traffic.

The inner workings of these boring looking cabinets are both surprisingly simple, yet very complex. In other words, the basic design is simple. There's just alot inside it!

The lights themselves are 110 volt, 60 cycle AC. The signal heads (lights) are provided in two sizes: eight , and twelve inch lens. Traffic lights use a special bulb designed specifically for the rigors of constantly being turned on and off. The eight-inch units use a 130-volt, 69-watt bulb, while the twelve-inch units use a 130-volt, 117-watt bulb. The reason for the 130-volt bulb is, that when operated on 110 volts, life is substantially increased. It is interesting to note, that a typical traffic light setup used for two directions of traffic, would operate on a single 15 amp, 110 volt service.

Now on to the stuff residing inside the ugly gray cabinet....

The lamps are controlled via a device refereed to as a "load switch". The load switch is a 100% solid state device, using SCR's (silcon controlled rectifiers) activated by a 24 volt control circuit. The 24 volt control circuit is handled by the "controller". The most common controller around is the TMP 390, manufactured by a company known as "Traconex". The controllers were at one time mechanical devices. However, these have all been replaced by computer controlled units, such as the Traconex model TMP 390.

Of course, since traffic conditions change and are fairly predictable, the need to alter timing of the lights during certain times of day becomes very useful. In other words, you need a programmable controller. The controller can be programmed in a couple of different ways. One is via the keypad and display on the front panel, located on the front of the controller. The other, is via a modem and the corresponding piece of software running on a PC. Of course, only controllers that are in high traffic areas are equipped with modems.

On busy streets, there will often be a string of cabinets, all controlled by a single master controller. The TMP 500. Virtually all master controllers are equipped with a modem. A simple piece of software exists, which enables someone with a PC to dial up the controller and alter the programming of the system (time of day programming, etc).

The controller is monitored by a "watch dog" called a "conflict monitor". The conflict monitor has a wire attached to each of the 24 volt control circuits of the controller. The purpose of this unit is to ensure that, for example, two green lights cannot come on at once. The programming of the conflict monitor can be altered, but only from within the traffic cabinet, via the monitor's front panel. Basically, a disaster avoidance device, the conflict monitor effectively stops the system from doing anything really stupid. It is virtually fool-proof, as it also monitors the high voltage side (the wires connected to the lights themselves). Again, if more lights come on at one time than is allowed by the monitor's programming, the intersection is brought into a "flash' state (flashing yellows on all signals). I should also mention that the conflict monitor's programming can be altered ONLY from within the cabinet by a person that is intimately familiar with such a unit. Power for the signal heads is routed though a bank of normally closed relays. When energized, these relays route power from the load switch (SCR device) to the signals. If the controller goes down for any reason, these relays default to the flasher circuit. This circuit is flashing at all times to avoid any delay that might otherwise exist. Flashing is accomplished via a solid state flasher of a similar design to that of the load switch.

However, there are also a few other factors that can alter a traffic light's behavior. Ever noticed that some traffic cabinets have an amber light fastened to the top? Well, some do. You may also notice that these lights are accompanied by an antenna. Of course the antenna is attached to a receiver which monitors for signals transmitted by emergency vehicles such as fire trucks, ambulances, and so on. Some city buses are also equipped with the transmitter. Upon receiving this signal, a "call" is placed on the controller. This, as far as the controller is concerned, has exactly the same effect as a pedestrian pressing the call button on the street corner. The controller then changes the signal to green for that direction of traffic flow.

One other thing that has the ability to alter the controller's behavior in such a way is a simple device called a "ground loop". This simple device consists of a few turns of number 14 gauge stranded copper wire inside pieces of plastic tubing assembled in a diamond shape and buried a few inches below the pavement's surface. The loop is attached to what is essentially a metal detector and senses the presense of a vehicle. (Does not work on rats, dogs, etc). The detector is fully adjustable, again only from within the cabinet itself, to detect anything down to the size of a bicycle. In northern climates, there are obvious problems with these ground loops. They are prone to failure due to moisture getting inside the plastic pipe, freezing and breaking the wires. Rockwell has developed a much better system for dealing with the issue of vehicle presence: The Rockwell Traffic Cam. This system consists of a low resolution camera and a simple, dedicated computer. With a dedicated piece of software loaded on any Windows based PC, one can access this computer and view and adjust "virtual" ground loops, or now known as "virtual detectors". This system, while much more costly, offers a far greater reliability and flexibility. A ground loop is only adjustable for sensitivity. The Traffic Cam can be adjusted for width, length, number of lanes, distance from the stop line, etc.

But alas! There is yet one more way that the members of the local law enforcement may effect the behavior of a traffic cabinet.

Ever notice that the main door on the front of the cabinet has a small sub door that is about two thirds the size? Well, if you didn't, you should! Behind this small door lurks a panel known as the "Police Panel". This panel is equipped with a row of switches that perform various functions.

These functions are as follows:
1. Advance. When toggled, this switch allows Officer Friendly to operate a momentary contact switch enclosed in a small housing on the end of a coiled cord tucked inside the Police Panel. When this switch is pressed, the traffic signals are forced to switch to the next "phase" on demand.

2. Flash. When toggled, the intersection is placed in either a flashing red or flashing amber state. The choice of red or amber is hard wired.

3. Off. This should be obvious.

4. Run. This should also be obvious.

As with so many things these days, there are security issues:
-All the cabinets use the SAME key!
-All the police panels use the same key. Be it a different style than the main door key, they are all the same.
-To connect to the modem, you must use an older Hayes compatible 1200 or 2400 BPS modem. Nothing else works. So, an otherwise useless modem has great value to the traffic control industry.


Hollywood would have us believe that it's possible for someone with a PC to shut down, or otherwise take control of the traffic control system of an entire city. Not gonna happen! It may be possible to alter the behavior of one particular bank of lights on a length of street. However, since the master controllers are not connected with each other, there is not way to alter the behavior of the entire system.

So, with this knowledge, it should become obvious that it is impossible to gain enough control over individual or multiple intersections to cause any changes of significant value, such as those depicted in the movie "Hackers". Once again, someone in Hollywood has not done their research.

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