Conversation:

A while back there was an article or news item about speeding up the traffic on the gyratory system. How did they do it? Well an engineer from the council thought he'd be able to solve the problem, so went and watched the traffic & the lights and worked out that if the traffic lights stayed red for longer, then traffic would flow faster. Guess what? It actually worked and although you'll be sat at a red light for longer, you'll actually get onto, across and off the system quicker!
Unfortunately, I can't give you a source, but I'm sure if you 'phone the council, they'll confirm.

A quick note on Swindon's Magic Roundabout. My sister went to college in Wiltshire's 'Windy City' & so I was an occasional visitor. My first piece of advice on how to tackle said roundabout is to avoid Swindon altogether. My second is to ignore all road markings as the (alleged) official line is that although it is one largish roundabout with (I think it's eight) mini-roundabouts arranged around the outside, you don't actually have to drive around the mini-roundabouts (not sure about the largish one though). Or alternately, just pray, take a change of underwear and make sure that whatever it is you're driving is larger than everything else.

Mark

Counter-intuitive traffic light timings are actually pretty common. Most of the major roads in London have pretty highly-tuned traffic light systems. A very clear demonstration of this is that whenever the traffic lights are down and a police person is brought in to direct traffic (still happens occasionally), he or she will almost invariably let traffic through for far longer at a time than the lights would have done. This gives the impression (to the person directing traffic) of reasonably smooth traffic flow, with nice long runs of fast-moving traffic. However it turns out to be an illusion - you end up with *much* longer tailbacks.

I think the main reason it causes problems has to do with the length of time sat stationary on a single phase. Short phases are what you want, otherwise you get too big a glut of traffic building up at the lights, so the traffic in effect has more momentum to overcome when the lights change. This actually means that you get more time which is effectively 'dead' (both lights on red - happens when one direction is being stopped before another is being let through), but the net result is a big increase in flow. (Although if you take it to extremes and don't let any cars through at all, it doesn't work... But I've seen some lights only on green long enough to let 2 cars through, and it wasn't just because they had radar like some lights do - it was letting just 2 through when more than 2 were waiting!)

The Hanger Lane gyratory might be a bit different though. I can believe there are considerably more complex effects to do with the volume of traffic actually on the system itself. So if they're simply choking the traffic a little on its way in to reduce the number of cars in rotation, I can well believe that this would make things go much more smoothly. (So in this case it's not the short-phasing thing that works well for most junctions, it's simply keeping the system itself reasonably clear. I'm guessing that the longer red-light times applied only to cars coming onto the system, not those being let off it.)

I find it amazing that London copes as well as it does with the staggering volume of traffic - most places grind to a halt with much lower densities. I think it's a combination of cunning traffic planning plus the fact that people are all used to driving faster and with less personal space in London...

I once used similar attack techniques every day when negotiating Marble Arch on a bicycle (yes, I was young and foolish then). I later upgraded to a Land Rover with a conspicuously dented front bumper, and this made life so much easier. Even taxi drivers treated it with respect.

The point about the timing of traffic lights is valid, except when the lights are temporary controlling alternate flow at road works. If these are set for too short a run, the queues build up rapidly. A good long run of around 20-30 seconds is usually best, as there is a long delay while waiting for vehicles to clear the single lane. Unfortunately, this does not always happen, and it is all too common for the lights to allow through just half a dozen cars (plus the usual few red light jumpers). More frustrating still is if the lights turn green too early, when the single lane is still full of oncoming traffic.