Been there, done that.
While driving a back road in France 24 years ago, I stopped for gas. After an unexpected big debate with the bumptious station owner (when he refused payment in French Franc traveler's checks) I was so vexed and rattled that I forgot my wallet on the car roof. Shortly after paying off the man in the currency he demanded, having driven off in a hurry to resume my trip, I heard a strange rustling sound. I glanced in the rearview mirror and saw something floating through the air. Funny, it looked almost like money. Wait a minute...it was money! It was my money! I slammed on the brakes and ran back to collect my cash, scattered all over the road.
How fast had I been driving? Surely no more than 50 mph.
It was a cramped little Renault 5, fully laden with 3 adults and luggage, so its acceleration and top speed were limited to begin with.
I was on a secondary road (one-lane, undivided) that wouldn't have supported seriously fast driving in any case.
The money started flying no more than 30 seconds after I'd pulled out of the gas station. Not enough time for this underpowered, overloaded car to have achieved serious speed.
The fact that I was close enough to the scattered cash to run back on foot and retrieve it suggests that I couldn't have been driving so fast.
Finally, I rely on my own memory: I can just recall that I wasn't driving fast. And the wallet itself was still on the car roof.
There's no way on earth these people could've been driving faster than 60-70 mph and hafe money flying from a wallet resting on the roof.