18 miles per hour? My average pace on a bicycle is 20 mph. This is hysterical.
You use a car when you _have_ to, not simply as a default. If it takes longer to get to your intra-city destination in a car, that's the city working as intended.
That's only a good thing if you accomplish it by making other forms of transportation faster and more direct. It's a bad thing if you do it by making cars slower and less direct.
I'm guessing that the improvements in efficiency (fuel, noise, wear) and safety are probably going to be worth it.
Also, I don't see cars being a fast way to get through the city with today's speed limits, I'd say my 6km commute in SF is faster on bike, especially after accounting for parking.