To do one green light suicide is to run across the side of the street that is currently open, press the pedestrian button and run back to the other side of the street and press that button. This can be done in hopes that the light will change faster. To measure physical improvement, see how many you can do before the light changes and keep trying to improve your score at the same intersection.
Oh god i'm so out of shape, I could only do two green light suicides before the light changed. The antonym for green light suicide is a red light suicide.