Why people don't use bicycles is difficult to understand. America may be 'special'. One interesting phenomenon has occurred in Paris, France. An enormous increase has been seen in that city since bike lanes have been augmented and rental 'cycles have become public and widespread.
Marion Morrison is completely correct in pointing out the importance of physical separation of such vehicular traffic.
Bicycles are the post efficient means of human transportation.
I bike. Not for environmental reasons, but for fun, and exercise. It also enables one to enjoy the scenery more than being caged up in a car. It allows more freedom than walking as wheels are a lot more efficient than just legs so you can go farther, faster, and get places.
What I am seeing now in cities is rental of E Bikes that can get people around without pedaling. Many Americans aren't in the kind of physical condition to bicycle. It is pretty sad, but true. People here are FAT in general.