8.4 Red Flag and Race Suspension — Sporting Regulations, Article 41

The Race Director will suspend the race (display red flags at the start/finish line and all marshal posts) when: There is an accident or incident of a dangerous nature that cannot be safely dealt with under a Safety Car; A significant portion of the track is blocked or compromised; Weather condit...

Source: Article 41.3

Send this to your friend — argument over.