Here's what we do locally in the regular (tues/thurs) interval session/motorpace race: http://lavelodrome.org/Training/RogerSession.htm

it works great because everybody gets the best workout they can get. the motor game is usually 40 to 60 laps, where the first 20 are "easy" and people who are strong can get laps. It gradually accelerates and then at about 20 to go it's too fast for anybody to gain laps, and keeps getting faster until 3-5 people are left for the finish. We also do motorpaced madison sessions on sundays, where it's usually 30 minutes at a constant high tempo and 30 minutes of gradually accelerating. The motorpaced madison is effectively an interval session.