Use your headlights when the weather is bad enough that you have to engage your windshield wipers. The reverse is also true: when you have to use your lights, don’t forget to wipe. Okay, that was childish.

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You should also use them any time you may have trouble spotting other drivers or they may have trouble seeing you. For example, if you’re on a small mountain road driving a Smart Car (so named because if you get hit while driving one, it’s really going to smart).