Verified answer

An old superstition dating back from the days when many cars had rust holes through the floor.  If you left your feet down, they were likely to be splashed by the water.   

I know what this is.  Your friend is an idiot.  

I'm Irish (well, part of me is), which means my people literally invented silly superstitions.  I've heard them all, because my grandmother actually knew them all.  She'd grab her Jesus-beads and yell about Hail Mary every time there was a thunderstorm, she knew what seeing three crows on a fence before lunch time meant, she knew all about the meaning of an undertaker leaving his hat on the kitchen table, she knew to hold her breath and cross her fingers when passing a cemetery, you name it she knew it.  And they all meant bad luck, of course.

And my old Granny, rest in peace, had never once heard anything about lifting your feet while driving over puddles and bridges.  And if she ain't heard of it, it ain't a silly superstition.  That's how I know your friend is an idiot.

He's a moron maybe?