> Honest question: wouldn't tipping be a major problem for half-width cars? And the amount of space needed between cars wouldn't go down much, so if you need at least 4 feet of clearance on either side, and your lane is 5 feet wide, your cars can only be 1 foot wide. This explains why most vehicles thinner than a car are motorcycles or electric scooters. 

Probably would be a problem. I don't know that the author was thinking "a car that is half the width of a normal car" as "a vehicle that takes up substantially less space". Perhaps we'll all be riding around in Go-Karts, who knows? :)