Baron58 is correct. It takes a few moments to stall even at a slow speed since these bikes have an anti stall "cut-in" feature for those that don't downshift to 1st at a slow speed. This feature prevents the bike from stalling and feeds a bit of fuel for a second.
And in sand I can see that back tire still maybe spinning and hitting and grabbing some sand as it is moving around a bit in soft material.
However, I can't see it lasting more than a second or two. And if you fell off at that speed you would like jump up out of shock and move away from the bike or try to pick it up.
In a crash at some speed (not sure what that is) the bike would cut off based on the tip over sensor, slide out and the emergency flashers would kick on. There is a picture on this forum of a bag that did exactly that. Just as it was designed to do.
And where the heck were you riding a Vintage and turning anywhere near sand? Belt driven bikes should not be any where near sand as it will throw it up on belt and damage your belt as it grinds it up with the pulley.
No if you were at the Bonneville salt flats. I can give you a pass. Don't ask me how I know!
