OK, all this is a long way short of proof that traffic density alone causes these differences.

BBC: Crashes

There are also huge differences in population density across the country, with the population per square kilometre ranging from nine in Eilean Siar (the Western Isles) and the Highlands to 3, 395 in Glasgow.

BBC: Census shows 'highest' Scottish population ever

But it is possible to detect minute deviations in this signal, and these fluctuations - seen as mottling in the map - are understood to reflect the differences in the density of matter when the light parted company and set out on its journey all those years ago.

BBC: Planck satellite: Maps detail Universe's ancient light