As a pracical matter is going to be the Police officer's judgement. If the Officer believes the car passed too close to be safe, the Officer can give the ticket. If he thinks the car gave sufficinet clearence to be safe, he's not going to ticket the car.
And if we're talking about applying the statute to establish negligence per se in subsequent litigation over an accident, it 's only going to come up when there's a collision, and then there will have been zero feet clearence.
It's not like there's going to be videotape, or scale photos to which one could put a ruler.
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You could fall off a cliff and die.
You could get lost and die.
You could hit a tree and die.
OR YOU COULD STAY HOME AND FALL OFF THE COUCH AND DIE.