Asked by: Andre Barker, Minehead
In a word: skill. The white 'paint' is made from thermoplastic resin mixed with titanium-dioxide pigment and tiny reflective glass beads. On major roads it's applied using dedicated road-marking vehicles that deliver the paint under computer control. But that still leaves the challenge of following the surveyed line precisely, and at the correct pace for the conditions - which is the task of the highly skilled (and highly paid) 'steersman'.
On smaller roads, the job is even trickier, with the paint being applied from a 'laying pram', a simple metal trolley fitted with a heater to keep the paint fluid. The skill now lies in both following the line precisely and at the right rate to ensure the lines are the right thickness and width - and also stopping and starting the flow to achieve the right pattern.
Geoff Ridd, managing director of Taunton-based RT Roadmarkings, estimates that only around 1 in 1000 people are able to master the technique.
Robert is a science writer and visiting professor of science at Aston University.