While watching racing (stock car, indy car, touring car, etc.), I notice that when the cars are following the pace car, they are constantly swerving left and right until the pace car leaves the track. Why do they do this?
If my race fan buddies are correct, they are “heating up the tires”, causing friction to get the tires to grip better in the race.
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It’s called “scuffing” the tires. Chandeleur is basically right. Swerving back and forth heats up the tires and also scrapes off the “mold wax” or oily coating that the tires come with from the factory, which improves traction.
Hot tires are sticky tires. Same reason that drag cars smoke their wheels before the race.
I had heard that it lets them drive faster. By swerving, they increase the distance that they travel, compared to driving straight. This lets them drive at higher speed than the pace car.
starfish - what??
Everybody else has got it. It has to do with evenly heating the tires, across their entire surface.
Generally if you see a pace car out there, it means they’ve been at yellow, which probably means they all pitted and got new tires.
When the difference between first place and 25th is about 1 mph, every little temperature degree and pound of air pressure matters.
My father worked pit at Indy for nearly a decade, so there’s my credentials. 
Basically: What Democrirus and Chandeleur said. They’re warming up, heating up, the tires to improve traction.
Something else to consider–those tires are about $5,000 apiece. Multiply that times the multiple pit stops–where all four tires are replaced every time–and you’ve got a serious expense, just for the tires!
There is also another reason to swerve back and forth. Say this is the second yellow flag in 15 laps and the leaders stay out to keep track position. When they slow down, the now hot, sticky tires will pick up small pieces of rubber from the track. If the don’t scrub these back off before a restart, the car is hard to control for the first few laps back at speed.