Steve,
You are certainly entitled to your opinion but...

 >And any impact sufficient to crack that tank and make it
>explode when the valve is open would crack the tank
>whether the valve was on or off

Sure, but I'd much rather have the flame off! That is the point of driving
with the valve off.

>besides, the impact would probably kill you first.

Not necessarily... it's possible a minor collision could crack the fitting
wher it comes out of the tank or rupture a seam/weld in the tank.

Anyway, you are certainly welcome to drive around with your fridge running
on propane if you like. And smoke too if you like. I just have a different
comfort about it. That's what I like about life... we all make are own
choices.

Good Luck
Bill