Traffic lights

"Only three lamps – easy!"

I always thought a traffic light was three bulbs. In reality a modern traffic light is a complex piece of software engineering.

The individual lights have software to control the sequencing of its light signals, the story doesn’t end there. Separate signals at a junction need to operate in a co-ordinated fashion, this bit is obvious. There is no point in your light going green at the same time as the crossing traffic’s light goes green.

The clever bit is when nearby junctions operate in harmony. This is achieved by the traffic signals communicating. This communication can be at a local level or at a city level.

The job we were asked to do was to allow the traffic lights to be reprogrammed without engineers having to visit every signal. On the basis that all the signals were chatting away to a central control centre, then why couldn’t the central control centre reprogram the signals when the telephone line wasn’t busy. Well it could.

The real problem was that the traffic lights chatted to the central computer every second or so. Also we couldn’t switch everything off for half an hour whilst the data was transmitted.

So the solution was to transmit small packets of information to each traffic signal during the small period each second when it wasn’t talking to the control centre with traffic information. This operation being enacted for dozens of signals at the same time .

This involved calculating the unused time each second, calculating the actually achieved transmission speed over the telephone line and then working out how many bytes of program could be transmitted in any second. The bad news is that traffic signals are connected with very cheap, very slow telephone lines (200 baud), a modern modem runs at 56000 baud, nearly 300 times faster.

It was done using a PC. The PC did not run DOS or Windows as this was too slow. A special purpose (very fast) operating system was written that allowed the flexibility we needed. In addition some special hardware was developed that allowed the PC to talk to many telephone lines in quick succession.

This was completed successfully and operated well within the agreed specification.