Railroad tankers and trucks carrying big tanks of it routinely go by the road right down the way from me. Maybe 1/8 miles max.
Still don't like that stuff. Spent 25 years with Volunteer FD and I was the ChemTrec contact discussion guy. Still generally subscribe to the one thumb gauge.
If you can cover the whole incident with one thumb, at full extended arm, you are far enough away. If not, move farther away.
Nemo