Sprints aren't intervals. The 20s/40s thing won't work.

If you want to do a sprint workout, find a loop or a long, straight road. Measure off 1000 yards. Paint the start and finish line with a rattle can, along with 200, 150, 100 and 50 yards. Put a cone at about 175 yards.

Start at the 1000 point and gradually build speed as you get near the 200 yard point. You don't need to be going 30+ when you get there, but 23-25 is good ballpark. Be in the gear you can sprint in and get on top of. When you hit the cone, JUMP with all of the explosive forward momentum you have. The jump is what wins or loses sprints. The ability to get on top of the gear quickly and stay on top of it.
Go as hard as you can, without taking yourself out, all the way through the line.

I do 10 of these. When you finish, roll back to the start slowly. It will give you enough time to recover for the next sprint.

Another exercise is to come, almost, to a complete stop. 5 mph or so. Put the bike in the 53/15 or so.
When you are barely rolling at 5 mph, try to jump and get on top of the gear. This forces you to use every part of the pedal stroke in order to get up to speed.

I weigh 175-180 and really don't generate numbers that are all that big (1500-ish for 5 seconds). I can tell you though, getting the jump has won me more than a few races and served me well in 35+ masters fields.

Race sprints start at 30mph +, but these exercises will help you get to where you want to be. I make my juniors do them and they all have huge snap now...