The start of the race is usually signaled to the driver by turning on and off the signal. However, the race can also be started using national flag. Even now, In case the signal cannot be used due to troubles, the race starts by waving national flag.
Regulations
Standing start
Usually, the start of the race is conducted using the start signal, but if the signal becomes anavailable due to the trouble, the start of the race is conducted using national flag.
After the formation lap, when all cars stops on the own grid, the starter raises the national flag overhead.
After that, when the national flag is swung down, the race starts.
Normally, the time between when the flag is raised and when it is swung down must be within 10 seconds.
However, the start using national flag is not conducted without notice. In the unlikely in case national flag is to be used, the race control usually informs the driver and team in advance of the starting procedure.
Rolling start
In rolling start, if the signal cannot be used due to some trouble, a race start using national flag is conducted.
After the rolling lap, when all cars form a two-row convoy and through the last corner, the starter signals the start of the race by waving national flag.
National flag is used in endurance races
Even in endurance races such as WEC (World Endurance Championship) and Super Taikyu, national flag is still used at the start of the race even today. However, the start is only conducted by signals. The national flag is used as an aid.
In 2014 Le Mans 24 hours Endurance Race, Fernando Alonso of Formula 1 driver waved the national flag (French flag) at the start and became a hot topic.
出典：as-web.jp
2.5.5.1 a) National flag
This flag may be used to start the race. The starting signal should be given by lowering the flag which, for standing start competitions, should not be raised above the head until all cars are stationary and in no case for more than 10 seconds.