For a modern braking system the 2 second rule is a generally a safe following distance. I don't see the need for a scientific measurement of the stopping distance. Allow a lot of extra room to stop until you know the vehicle well enough to tighten up your following distance.

1953 Chevrolet 3600
Fried Green T'mater (Vern)
In the Stovebolt Gallery
Caution: Political Statement
Man who runs in front of car gets tired, man who runs behind car gets exhausted.