A solid yellow flag is used to slow down the racers in the case of a crash, debris on the course (like car parts), slick fluids on the track or the weather has become a factor with rain. During a yellow flag the pace car, with the top lights flashing, joins up with the race leader and sets a slower speed on the track. This is done to preserve the driving order on the track as cars cannot pass one another under a yellow after crossing the start/finish line. Drivers generally use this time to hit the pits for refueling, new tires and adjustments. It is an advantageous time to make the race car faster on the track.