Every extra person driving a car adds to the total risk to everyone. I'm of the understanding that "safer" driving is a function with an asymptote well above z

Every extra person driving a car adds to the total risk to everyone. I'm of the understanding that "safer" driving is a function with an asymptote well above zero, i.e. there's only so much that a person's skill as a safe driver will improve the net safety of the system. I'd like to add that I'm not just talking about the safety of the extra person driving; there's also the safety of the other road users, whether in cars, on motorcycles, on bicycles, or on foot. An extra driver on the road raises the net risk for everyone. Additionally, in my experience the intersection of "drives for fun" and "drives safely" is so close to zero that it's not worth considering.