Bull dog
Its usually because your looking around at the scenery or at stuff, then notice the green arrow come on. So you figure, if they can turn, then maybe yours turned green also, so you step on the gas. You do all that in less than 1 second. Its out of driving habit.

jtexas
Cause at our core, we're pack animals.

Matt A
It's a matter of instinct. People have gotten so fixed not only on symbols but also on colors, it's what defines different objects. As the other person said, they can be looking around and just catch a glimpse of green and they're foot reacts soon after the brain but the eyes are slower to correct the foot (that made sense in my head). If people take driving not as a method of going somewhere fast but as a fun leisurely adventure, this reaction would be reduced.

lar
The reason I do it is because I'm always afraid the light will turn yellow or red while I'm in the middle of it and I've gotten a ticket for running a red light before.

Wayne O
I only do that when a ricer is next to me. Its a habit left over from the old days of hot cars and hotter women.

archerdude
Mostly because they are too busy thinking about how they "have to" get somewhere "right now", or some other not-really-so-important thing, and they simply do not bother to PAY ATTENTION to their driving (which just happens to include paying attention to traffic signals and signs). When asked by the police officer how she happened to drive her Mercedes into the side of a Mack truck pulling a belly-dump trailer, the woman replied "I was looking at the roses at the side of the intersection, thinking about my own rose garden, when I saw a green light out of the corner of my eye. I thought it was the signal for me to proceed and I just naturally stepped on the gas...."