Why did Mike drive himself when he went to visit Hymen Roth?

Because he needed to free his bodyguard to look out on all sides to make sure assassins weren't following or converging. If the bodyguard drove, he'd have to concentrate on the road ahead.

Turnbull;

I was under the impression that Mike drove to use the bodyguard as a decoy, much the same way that they put Frankie Pentanglies in an Army uniform. The thinking is that if there was an assasination attempt, they would go for the "guy in the back seat"

I figured Buscetta was from Italy and he didn't have a license.

By TBs explanation, would that mean every Boss would drive and have their bodyguards watching out for assasins?