Yellow, Green, and Red

I have not been a licenced driver for an exorbant amount of time, which, once you learn the product of my observation, presents this question: "Is it just me?". I have noticed, time and again, that when one is driving along and they come to a traffic light there are always four possibilities . . . one, you might have to stop, two, you may be forced to slow down, three, you may be given leave to keep going, four, you may be given leave to keep going. Number one applies to a red light, number two applies to a yellow light, number three applies to a green light, and number four applies to a yellow light (in most cases). When most people come to a traffic light and it turns yellow, they generally speed up to get through instead of slowing down. I don't know why, but I really do find that humorous. From the beginning of your life you learn that red means stop, green means go (according to my drivers ed teacher, green actually means "proceed with caution" . . . I don't know about that one), and YELLOW generally means slow down . . . even Barney says so! Anyway . . . I'm not quite sure where this one came from.