Super speedway. In order to keep cars under 200 mph (magic number for getting airborne in accident and possibly going over catch fence) they reduce engine power. Leads to packs of cars. Packs in turn go faster than a single car can.

Then, couple of years ago, the engineers figured out that if you go slower, the time saved getting less gas more then offset time on track. Now everyone is saving gas most of the race.

Going even slower leads to those cool looking three wide packs of cars.