Went for a group ride last weekend. Mixed group of men and women and different abilities. Usually 15 to 16 average speed. Two racers showed up and decided we'd practice pace lines. Their choice was to have the line maintain speed, and the last person accelerate to the front. One person in the group would not slow down once he got to the front, so every circuit, the group had to catch up, and the person on the left had to really push to get in front of the guy. Pretty soon we're doing 18 to 20 mph. Not fun.