|Illustration in school text book, 1951.|
After the second world war, this problem of empty crossroads and waiting traffic was addressed by the installation of vehicle detection systems at all of the 5,500 traffic lights in Great Britain. At the approach to each set of lights a pneumatic pad was set into the carriageway. The weight of a vehicle crossing it closed an electric switch and thus influenced the sequence of lights. At the green light, if no vehicle crossed the pad for the space of twelve seconds, the lights changed to red. By the late 1960s, poor maintenance of the two systems used (GEC and Plessey) meant that 50% of the traffic lights in London had reverted automatically to the default of the pre-set timed sequence. A new solution was needed and thus the induction loop system that we use today was brought in.
|The grey lines on the carriageway are the wires of the induction loop system which detects the presence of vehicles waiting at the traffic lights.|
Visiting Folkestone today, I noticed in the pavement next to the induction loops, a metal utilities access cover.
This was all that was left of the electro-pneumatic switchgear which formerly had controlled the detector pad for the traffic lights.
However, on the other side of the junction, the disused pneumatic pad could still be seen in the carriageway.
|A bit of traffic light archaeology: the pneumatic detector pad introduced in 1947. It consists of two strips, isolated one from the other so that it can decide the direction of travel of the vehicle.|