Stopping distance. The car travels at the speed limit for the reaction time — 1.5 s is a normal alert driver, not a distracted one — then brakes at the friction limit of the tyres: distance = v² ÷ (2 × μ × g). Dry asphalt is about μ = 0.7, wet about 0.4, ice around 0.15. Real cars have ABS and real drivers brake harder or softer than the limit; this is the physics floor, not a guarantee.

Visibility. A dark costume is only picked out by silhouette and movement, and a driver on dipped beams sees it at roughly 15 m. A light costume catches more light, around 30 m. Retroreflective tape returns the headlight beam straight back to the driver, so it reads much further: the estimate here grows with the square root of the reflective area, from about 25 m at 5 cm² to over 60 m at 100 cm², capped by how far dipped beams actually light the road.

Why area beats brightness. Retroreflectors work by geometry — a patch has to be big enough for the driver's eye to resolve at distance. A strip 5 cm wide across a coat hem and cuffs, plus a band on each ankle, is far more effective than a tiny bright badge. Ankles matter because the moving pair of reflectors reads as a person, not a roadside object.

What this does not model. Glare from oncoming traffic, rain on the windscreen, dark clothing against dark hedges, parked cars, hills, bends, and drivers who are tired or on a phone. A torch helps the driver see the child and helps the child see the kerb, but it also ruins the child's night vision. The best fix is a route with pavements, crossings and lower speeds, not a brighter costume.

Last reviewed: 2026-10-20. A road-safety screening aid, not a substitute for the Highway Code, local speed limits or adult supervision.