[Hardware]

There's no clear-cut threshold you can explain. It varies depending on a wide array of factors. The configuration of the intersecton & roadways, the speed of the approaching truck, adjacent stopped vehicles and moving traffic in the vicinity.

That said, err on the side of caution and use your escape route (you always leave at least one, RIGHT???) earlier as opposed to later.

Better to ride up onto a sidewalk early, find out that the truck was actually able to stop safely well behind your stopped position - and feel a little silly - rather than have EMT's remove you from the undercarriage of a Kenworth with a steam cleaner...