Pennsylvania Vehicle Code Law: (edited for this website; most states have similar laws in place)
4302. Periods for requiring lighted lamps.
(a) GENERAL RULE.-The operator of a vehicle upon a highway shall display the lighted head lamps and other lamps and illuminating devices required under this chapter for different classes of vehicles, subject to exceptions with respect to parked vehicles, at the following times:
(1) Between sunset and sunrise. (but turn on ALL your lights when the sun is low in the sky - morning or evening!)
(2) Any time when the operator cannot discern a person or vehicle upon the highway from a distance of 1,000 feet due to insufficient light or unfavorable atmospheric conditions, including rain, snow, sleet, hail, fog, smoke or smog.
(3) Any time when the vehicle's windshield wipers are in continuous or intermittent use due to precipitation or atmospheric moisture, including rain, snow, sleet or mist.
(b) SIGNAL LIGHTS.-Stop lights, turn signals and other signaling devices shall be lighted as prescribed in this title.
(c) APPLICABILITY.-This section shall not apply to motorcycles.
What this means: Daytime running lights DO NOT meet this code! ALL lights, front, rear and side lights MUST BE ON!
Know your vehicle. DON'T RELY on automatic "ON" for lights to come on. Often times the front windshield magnifies the outside ambient light and PREVENTS them from coming on when they should, plus your "auto on" doesn't know when it is raining, foggy or the sun is too low on the horizon.
Don't count on your dashboard lights either. Just because they are on does not mean ALL your lights are on.
Don't just use fog lights either. The law says ALL other lights have to be on.
Motorcyclists don't have sun visors and often have to look through several layers of plastic (sunglasses, face shield of the helmet and a windshield) in order to see.
Vehicles without all lights (front and back) on disappear under these types of conditions. Having ALL your lights on allows a motorcyclist to SEE YOU COMING AND GOING!
ALL LIGHTS ON - ALL THE TIME! It's the educated and responsible driver thing to do.