Even with bright lights and everything else working its can still be the battery. It takes more amps to turn the starter than any other light, radio, or accessory combined. A corroded post is your precursor to a dead battery. Keep them clean if you want them to charge properly. If the posts are tight but corroded it can still be enough to hamper proper charging from the alternator and starting of the car, the battery deep cycles and dies. When at a light the alternator may have not been enough to keep the car running as the rpm dropped to idle. Just my two cents...