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A PHYSICIST WRITES . . .
(September 2004)
Here is a selection of motoring mysteries:
Every modern car is equipped with seat belts and head restraints, and many also with air-bags, crumple-zones in the bodywork and other safety features. But why is there nothing to protect my head from the hard surround of the sunroof an inch above my head? A well-known consumers’ organization tells me that when testing cars and rating them for safety, they ignore the potential danger to the cranium from vertical impacts or shocks. I think some heads need testing.
There are never-ending arguments in the media and on the internet over the imposing of speed limits on roads that possess no obvious hazards. Why do I never see or hear it suggested that the purpose of a speed limit, partly at least, may be to indicate the presence of hazards that are not obvious (except perhaps to advanced drivers)?
What is it that creates the moving patterns you often see as you drive alongside railings or approach overhead foot-bridges and motorway gantries? Answer: they are examples of the Moiré effect. The word describes the shimmering effect of silk, and is not the name of a French scientist as I used to think (and as most other scientific labels seem to be)!
The effect occurs whenever two identical or similar patterns are superimposed. Scientists make use of it to reveal very tiny movements and patterns — even the arrangements of atoms in crystal structures.
To save me drawing diagrams: take a pair of pocket combs, hold them up to the light and experiment with moving or tilting them across each other. The lighter parts of the pattern are where the teeth cross or overlap, and the darker parts are where they happen to lie in between each other, blocking the light.
At the roadside or near a bridge, you are probably seeing two sets of identical railings but at slightly different distances from you, which adds to the effect. Sometimes the Moiré lines appear to be travelling alongside you at the same speed as you are, which can be a dangerous distraction.
On some overhead gantries the matching patterns in the structure create intricate two-dimensional Moiré displays, which I find equally distracting. You can observe such patterns much more safely by folding a handkerchief in two (it doesn’t have to be silk!) and lighting it from behind.
How quickly do the warning signs for new road layouts have to be removed, after completion of the rearrangement? Answer: the regulations actually say three months — but from my observations, the signs generally stay up for at least a year.
This means that what you really have to look out for, on a familiar road, are new signs for new road layouts. On unfamiliar roads, the only value that the signs have is as a warning to you that other drivers may think they know the layout but then discover too late that they don’t.
Early this month a man was imprisoned for causing death by dangerous driving, having been distracted by using his mobile phone. How did the BBC report this cautionary news? By showing us a reporter driving along the same road while talking over her shoulder to the camera.
Peter Soul
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