The Single Headlight Theory

Driving home tonight I saw four cars with broken headlights. Me being me, I started to wonder why,

I've decided that it is the cold weather. Turns out that the electrical resistance of metal falls when the temperature drops. The lower the resistance, the larger the amount of current that can go through the filament. The greater the current, the more chance of the lamp failing if it has developed a weak spot somewhere. 

This is why bulbs often go "pop" just as you turn them on. The peak of current that they receive before the filament has warmed up and started to glow is usually when they are going to fail.

Up until now we have had a pretty mild winter, so lights with weak spots have had it easier. But over the last couple of days it has been proper cold, and I reckon that the extra current that has gone through the bulbs has finished them off.

That or I just noticed them more....