When cars start from the pit lane, what determines when they can start?

Question

When an F1 race start (or re-start) takes place, what determines when cars starting from the pit-lane allowed to start? Is it based on cars on-track or something else?

I was thinking of this particularly in regard to the very unusual race restart in the 2021 Hungarian GP where every car except one came in to change tyres, leaving Lewis Hamilton as the only car on the grid with all others starting from the pit lane. If Lewis suffered car failure at the start or simply decided to sit on the grid at lights-out would everyone else have been allowed to restart?

Answer 1

There's a separate light signal at the end of the pit lane, that shows green if cars allowed to leave the pit lane, and red if they're not. Cars that start the race from the pit lane have to wait for that light to change from red to green before they can set off. This is the same procedure that occurs at the start of practice and qualifying.

My understanding is that the light doesn't change automatically, and has to be manually triggered by the race director. This ensures that cars can only leave the pit lane once it's safe to do so (i.e. every other car has passed the pit exit, and there's no danger of crashing into someone while merging onto the track).

The scenario you describe would most likely lead to an aborted start as soon it becomes clear that Hamilton can't (or won't) move. You can't have cars stranded in the middle of a live race track.