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A PHYSICIST WRITES . . .

(February 2009)

This month, several things Iíve written about in recent columns are still on my mind:

In November I remarked on the subtly different appearance of LED (light-emitting diode) lamps at the rear of some newer cars ó their indicators and brake-lights cutting in and out instantaneously, instead of rapidly but visibly warming up and cooling down as filament bulbs do. Afterwards I started wondering if the tail lamps on these vehicles were also LEDs. This isnít easy to tell, of course, because you donít usually see them being switched on and off.

Then one evening on the road, I must have shifted my gaze rapidly but Ďsmoothlyí, because all the lights in front of me briefly smeared out into tracks, except for two of them, which appeared as lines of dots! I realized that I was looking at a pair of LED rear lights being pulsed (by their built-in power supply) many times a second. But what would be the point of pulsing them?

My immediate thought was that the same LEDs could then be used more brightly as brake lights, by being powered continuously. Since then Iíve quite often noticed LED tail lamps, including some that are certainly doubling as brake lights in this way ó but others also that have separate brake lights alongside. And sometimes the latter are LED too, sometimes ordinary incandescent lamps ... this variety and mixture of illuminations makes it look as if car manufacturers are launching new models that are only half-developed (as I suggested in November). Hereís another curiosity Iíve noticed: our local buses have LED winkers behind and filament ones at the front!

I have now learnt that LEDs are much more widely used in N America, both on vehicles and in fixed lighting in the street. But thereís a problem, apparently: because of their lower heat output, LED street lamps and traffic lights tend to clog up in heavy snowfalls, instead of keeping themselves snow-free. And another thing: I understand that all LEDs tend to fade significantly over their (longer) lifetime, whereas filament bulbs usually burn out while still close to full brightness. So maybe LEDs donít represent unqualified progress.

I would think that small LED units are cheaper and easier to manufacture (in proportion to their size) than large ones, so that it probably makes sense to assemble them in an array or matrix in order to produce a large lamp. Already in the UK, many temporary traffic-light sets seem to be LED-based, and you can see what looks like a matrix when you get close to them. Recently too Iíve encountered new sets of permanent traffic lights using LEDs. I canít help feeling, though, that traditional warmly glowing filament bulbs exhibit a little more, well, politeness than these glaring instantly switching LED lamps which, when they change, seem to be telling you to stop, or else to go, RIGHT NOW!

I hesitate to return to last Julyís column and the question of how to approach a red light when you are expecting it to change shortly. Perhaps rashly, I suggested keeping some momentum going, and aiming to cross the line just as red-and-amber changes to green (this is assuming that you have a clear approach to the line). I could also have mentioned that the earlier you slow down, the more speed you will be able to retain, enabling you to make even better progress afterwards.

But I certainly ought to have added a warning not to put yourself on a collision course with red-light jumpers on the road that crosses yours. And if you manage to time your approach to the line perfectly when you happen to be carrying passengers, donít be startled by their screams because you seemed intent on barging through a red light yourself.

In a similar vein, there are occasions (for example: when arriving at an ordinary busy cross-roads) where you can delay a go/no-go decision and increase the chances of its being Ďgoí, by keeping some rolling momentum towards the line. Or you might choose to stop some way behind it and then roll forward just in case. Well, the danger to beware of here is that you will frighten other drivers into taking avoiding action. As for your already nervous passengers, they may decide to eject...

Last month, you may remember, I wanted to know if the 100 metre marker posts on either side of the M25 were allowed to get gradually out step as the motorway was laid out around London, from Dartford. The geometry of circles told me that the shift ought to be more than 200 metres by the end of the motorway (ie, when it arrives at Thurrock). Only later did it occur to me to look at aerial photos of the M25 on the internet.

The best ones I found were ĎBirdís Eyeí images at www.multimap.com: vivid perspective landscape views from all four compass points. But alas, their resolution isnít quite sufficient to reveal the posts. So I would still be interested in any observations from Thurrock!

[I now know the answer to the M25 marker question, and will reveal it in a later column.]

Also last month, I mentioned a warning I saw in an official manual for traffic authorities, against installing either of the two double-arrow Ďpriorityí signs upside down, in order to indicate the opposite priority. Only days after I wrote this, would you believe, I spotted a round Give Way sign mounted the wrong way up (black arrow pointing up, red arrow down) ó apparently by accident which ought to be an even more serious matter. This was on a diversion route around Petworth in W Sussex. It embarrasses me because presumably the sign has always been awry, and I must have travelled the road more than once without noticing.

A few miles further on, I saw a Stop sign in a little side road, facing sideways ó and then on the A29 a much bigger junction sign pointing exactly the wrong way. They were both on single poles, obviously, though I canít imagine what sort of sense of humour was satisfied by giving them a whirl. I reported all three signs to West Sussex County Council, but checking them again this month I found that the first two still hadnít been turned around, while the third one had disappeared, along with its pole. What is going on, down south?

In Surrey on the return journey, there was another curious sight: a set of traffic lights (the usual more polite sort) whose green lamp had lost its colour filter and was therefore showing white. Appropriately for the prevailing weather, it seemed to be saying GO, BUT WATCH OUT FOR SNOW!

Peter Soul

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