Nettled Shin wrote: ... Perhaps some technical article on traffic lights technology could be produced. The ones near me don't quite match thirdcrank's description. The channels cut in the road form a cross shape, and there is only one cross, with perhaps 3m diagonals.
There is a least one Traffic Advisory Leaflet that goes into this in some detail. I used to get the paper version before everything went online, and I have one on traffic lights that is more of a dossier than a leaflet. That's the place to look because if you are talking to highwaymen about something like this, it's handy to be able to talk their language.General principles of traffic control by light signals Traffic Advisory Leaflet 1/06 (part 1-4) http://www.dft.gov.uk/publications/tal-1-06/
I contacted the county highways dept a few years ago about traffic lights that wouldn't trigger. The response alluded to the fact that modern bikes have less and less metal in them, so the demand pulse isn't always strong enough. At a guess, there is a large loop of ac current-carrying wire buried in the road that has its inductance changed when a vehicle enters it. It would make sense that steel bikes create a stronger effect than a bike made from non-ferromagnetic material. If you have a carbon bike, I suppose you are more or less relying on your bottom bracket being detected?
It's called fobbing off. (That' expression relates to people, not traffic lights.
The guest speaker at the very first cycling consultation meeting I attended in Leeds was one of the assistant directors. It's sad to reflect that the subjects people raised with him such as potholes are still the big issues for us. He was asked about traffic lights and I can still remember his response, almost verbatim "I'm not convinced that the modern bicycle contains enough ferrous metal to do it."
I look for evidence, not myth, so I tested a newly-installed set of lights which I knew was very responsive. I found that using a drinks can, the ferrous and non-ferrous types worked equally well. As to quantity of metal, I found that the ferrule on a walking stick also worked every time. Convinced that the answer lay in adjustment rather than the technology, I contacted a big company specialising iin traffic lights who happened to have a big place locally, and spoke to a technical person. He confirmed that a transponder does not differentiate between ferrous and non-ferrous metal. He told me that the detectors were adjustable and can easily pick up a bike. The adjustment can be lost over time and so needs to be reset. (I later discovered that the Council's standard contract for light installation and maintenance included the ability to detect pedal cycles.) He also told me that the hardest challenge was the large lorry whose wheels were so far apart that they bridged some detectors and whose bodywork was very high. With all the safety grilles fitted to modern lorries to avoid cars going underneath them, this mainly relates to miltary trucks with a high ground-clearance.
There are obviously other types of detector and there must have been innovations since I was closely involved in this.
As I have said above, poor detection can be masked by other traffic, especially at peak times. Working shifts enabled me to research problems riding home at 0200 etc. At one consultation meeting I as was as good as dismissed as a liar when my info was 100% rock solid (I had done the testing myself in the middle of the night.) I only suffered that indignity once.