why does heat shimmer disappear from roads as soon as your line of sight is barely over horizontal to it?
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What you’re seeing is differences in air density caused by updrafts and downdrafts over the hot ground. The actual air patterns, which you can’t directly see, look something like a [Rayleigh-Taylor instability](https://en.wikipedia.org/wiki/Rayleigh%E2%80%93Taylor_instability) – you can imagine a heat shimmer as something like “anything in the purple bit of that diagram gets deflected a bit”.
Since these updrafts form mostly vertical columns, the number of different columns you go through depends on your horizontal angle. More horizontal lines of sight and more distant objects pass through more columns and therefore are more distorted, while more vertical lines of sight and closer-by objects pass through fewer and are less distorted.
Hot air over the road bends distant light up so it terminates in your eye instead of the pavement.
Because the phenomenon is localized and close to the surface of the road, you can often move your head a short distance to where this reflected light misses your head altogether.
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