In South Africa we were told it was illegal to drive barefoot and with any loose fitting footwear like slops. Probably why I have never attempted it.I would never drive in flip-flops because......
They flip-flop around
Bare feet certainly, as that gives you far more sensitivity than any shoe can. Competition drivers use special driving shoes that have very thin suede soles, for exactly the reason just given, better sensitivity.
However I believe that it is illegal to drive barefoot in Spain? Can't see any logical reason, but what do I know?