Yellow flags and yellow lights along the Circuit indicate the presence of an accident. Or, as it is called in the racing world, the shunt.
11 seconds sound clip from the Le Mans (1971) movie soundboard.
You can hear this line at 00:17:41 in the Blu-ray version of the movie.
Quote context
[...]
- His blood type must be indicated on either suit or crash helmet.
- No one may drive more than four consecutive hours.
- For the first time in the history of this race...
- Yellow means danger. No overtaking.
- The race will start 4 pm. Saturday and finish 4 pm. Sunday.
- The car covering the greatest distance in 24 hours shall be the winner.
- In the case of the faster cars, this will mean a distance in excess of 3,000 miles.
- And so, ladies and gentlemen, welcome to the most famous motorcar race in the world.
[...]