Just to clarify for anyone who doesn't know what clipless pedals are, Dan's describing a system where there's a special cleat on the bottom of his cycling shoe which connects securely to the pedals on his bike. When he wants to disengage, he simply rotates his heel out away from the bike and the pedal releases. Yet, while he's riding, his feet are secured to the pedals for greater power delivery. Clipless pedal systems were originally designed like ski bindingsa secure connection when you're upright, a quick release when you're going down or want to stopand they're standard in pretty much any professional race.
Now, back to Dan's question: A lot of people struggle to get clipped in when the light turns green, but there are a few tips that can help. When you know the light is going to turn green within a few seconds, get your butt on the saddle by raising up onto the toe of the foot that's on the ground. Your other foot (the one that's already clipped in) should be at the two-o'clock position. Being in this position allows you to start rolling forward and simply lift your foot on to the pedal.
Why is this any different than starting by standing over the top tube until the light turns? Well, try both ways in a parking lot and you'll see that you're much more stable and straight when you're butt's already in the saddle. Fewer things are moving, fewer body parts have to be coordinated in space, and most importantly, your weight is already supported by the saddle.
When you try to clip in while standing up, your bodyweight adds another variable that's hard to control in the heat of the moment. And, as Dan is experiencing, once you're standing and searching for the pedal, you can't keep pedaling with your other foot. When you start seated, you can pedal with one leg for a few strokes if need be until you get that other foot clipped in.
A seated start is a bit calmer, which often means it gives riders a better chance of hitting the pedal on the first try and getting up to speed quickly. Give it a try. Stand normally until the light is about to turn green, then raise up on to your toe and scoot the bike forward so you're sitting on the saddle with your weight on your toe.
When the light turns green, look forward and push down with your clipped-in foot. DON'T PUSH OFF WITH THE FOOT THAT'S ON THE GROUND. Don't use that foot to get you moving; you have another leg that's already clipped in that can do that. Just lift your foot as your roll forward and you can get it on top of the pedal by the time it's ready for its first downstroke. And you're off!