In addition we live on a (mostly) 2-dimensional plane so we can't scale transportation capacity indefinitely, which means even if the roads were owned privately

In addition we live on a (mostly) 2-dimensional plane so we can't scale transportation capacity indefinitely, which means even if the roads were owned privately it's nearly impossible for a market to form that would force them to compete with each other. Why? You can't build a close competitor to 9th Avenue (making up a street name) because A) there are buildings in the way beside it and B) building above it is prohibitively expensive. If these are one-way streets as is common in urban areas 9th Avenue's closest competitors are 11th and 7th and after that competition falls off quickly. Five competitors doesn't make a healthy market - it makes an oligopoly.