At the dragstrip, a street tree illuminates the three yellow lights in 0.5-second intervals and the green another half-second later. A pro tree illuminates all the yellow lights at once, then the green 0.4 seconds after. A perfect human reaction time is about 0.4 seconds, so on a street tree you want to leave just after the final yellow light; on a pro tree, go when the three yellows illuminate. Don't watch for the green light, as you'll be too late; keep an eye on the bottom yellow light for your signal to go. Roadraces may begin with either a flag or light, and it's best to check out a few races before your own to see how your particular club's starter operates. For lights all you can do is watch the light itself, but on a flag start, watch the starter for a tell-tale such as the tip of the flag moving, the starter's elbow bending or the like.