The answer is a lot. Not only do citizens share the cost of the construction and maintenance of ever wider roads, parking facilities, and all the infrastructure that comes along, but there is a huge opportunity cost to building cities for cars rather than people.
In the beginning, private motorcars were feared and despised by the majority of urbanites. Their arrival was seen as an invasion that posed a threat to justice and order. Drivers who accidentally killed pedestrians were mobbed by angry crowds and convicted not of driving infractions, but of manslaughter. At first, all levels of society banded together to protect the shared street. Police, politicians, newspaper editors, and parents all fought to regulate automobile access, ban curbside parking, and most of all, limit speeds to 10 mph.
I don't want to romanticize the past - particularly the angry mobs or banning of curbside parking (which is great) - but it's clear that even in car-centric cities there was a precedent for more equally and safely shared street space. Forward thinking cities around the world prove that embracing many forms of transportation in their streets can work today, without the horse poop. The low hanging fruit in this transition may be right in front of us. For example, just watch below how snow reveals all the extra road space that cars don't actually use or need.
Only when we begin to account for those costs and opportunities lost can we have a productive conversation on the cost of other forms of transportation. So when the topic of transit comes up at the dinner table or town hall, you know what to do.
In this video, I show some great shared streets in lovely Cambridge, UK and what happens when cities are designed for cars instead. Finally, urban scholar Dr. Yves Bourgeois weighs in on what's often missing in our transportation debates.
NOTE: I could talk a lot more about this, especially with the complex socioeconomic considerations, etc. I'm always wary of getting too wonky in the videos and blogs which are committed to a broad audience. That said, happy to dig into the complexities in the comments!