Stone chips from lorries: why the gap matters more than speed

A stone thrown up by the rear wheel of a lorry travels at the sum of its own speed and yours. At ninety kilometres an hour that means an impact with energy an ordinary windscreen survives only if the stone hits it flat. Hit by an edge and you have a chip.

What decides is not your speed but your gap. The stone follows a ballistic path and loses height as it goes. At a two-second gap it strikes the upper third of your glass, where it is thickest. At half a second you take the hit right in your field of view, and there we no longer repair.

Winter and spring carry the highest risk, because grit stays on the verges. After the first heavy rain the water washes it back into the lane and for the next fortnight cars recycle it between themselves. That is when keeping your distance on the motorway is worth it, even if it slows you down.

One more thing people underestimate: keep your distance when overtaking too. Most of the damage we see on the left side of the glass happened at the exact moment the driver pulled back into the right lane just ahead of a lorry's rear axle.

If you already have a chip, count in days, not months. Dirt and water have not had time to get into fresh damage and the repair is almost invisible. After a few weeks there is grime inside it that the resin will not cover.