Driving is not error prone, cars rarely break in unexpected ways.

People driving and making decisions are error prone.

A simple test is to watch how people turn. Do they turn early potentially hitting the curb or cutting it too close to pedestrians. Or do they increase their radius by turning late? The latter are better drivers.

Edit: here are more tests,

- do they signal

- do they cutoff others

- do they let those who signal in

- do they drive too slow or too fast for the given road and conditions

- do they have an awareness of all cars around them

- do they block the passing lane

- do they maintain a reasonable distance behind other cars

- do they let emergency vehicles pass

etc.