Because many cars light the gauges all the time, which fools drivers into thinking their lights are on at dusk. On older designs, daytime running lights leave the tail lamps dark. Transport Canada changed the lighting standard so newer cars with an always-lit dash must turn on the tail lamps with the running lights, or switch lights on by themselves in low light. On any car, check the switch.

It adds a second attack. Sun breaks down the coating in summer, then sand, gravel and salt spray scour the weakened plastic all winter. A restored lens with a proper hard coat holds up far better than one that was only polished. Parking in a garage helps. So does rinsing the salt film off the lenses when you clear the snow.

When the damage runs deeper than sanding can reach. Crazing through the plastic, haze on the inside face and cracks from stones all mean a new headlamp. Gravel roads can pit a lens so deeply that polishing leaves it wavy. In Ontario, a crazed or fogged lens that spoils the beam fails a safety standards inspection anyway, so replacement is the honest call.

Yes, noticeably. A foggy lens throws light sideways and up, so less lands on the road and more hits oncoming eyes. That matters on unlit highways during long winter nights, when you need to spot wildlife, black ice and snowbanks early. Keep the lenses clean too, because road slush dims even a perfect lens within a few kilometres.

They can. Ontario's safety standards inspection, which you need before plating a used car, rejects headlamp lenses crazed, clouded or fogged enough to spoil the beam or throw glare at other drivers. Most provinces also inspect a vehicle brought in from another province. Restoring the lenses before that visit costs less than going back a second time.