I've sometimes wondered what the exact rule is with red lights. Presumably you're allowed to continue moving forwards if the front of your vehicle passed the stop line before the light turned red. But if you stopped with the stop line passing through your vehicle are you allowed to start moving forwards again while the light is red? (Whether that would be a sensible thing to do would depend on whether you're driving a long vehicle with just a tiny part of it behind the stop line, or a motorbike with just a tiny part of it in front of the stop line, but does the law distinguish those two cases?)