Correct answer: When it has already snowed.

Put chains on when the roadway is already covered with snow, before the car starts to slip, or when a sign or traffic officer requires them. If it is only starting to snow and the asphalt is still visible, you do not need them yet. Put them on somewhere you will not obstruct anyone or put yourself in danger. In the image, the driver fits a chain with the road already covered in snow.

Nota de prensa del 26 de diciembre de 2025: Casi 14 millones de vehículos se desplazarán por España hasta el final de las fiestas navideñas · Manual II. Cuestiones de Seguridad Vial, edición 2026