French govt requires all car owners to buy one and keep it in the car for when the vehicle is disabled etc. It signifies that these are all middle class people out there protesting taxes.
It's even a bit stronger than that - from a UK driving organization for visiting France in your car -
Reflective jackets (one for each occupant, these must be kept inside the vehicle within easy reach)
With a yellow jacket for each occupant there are certainly plenty to go around...:^)