[aout]

I think the author doesn't want to mix the different types of transportation. He allows them to be mixed but users won't because it's not convenient. That does not mean it is not good. In Paris we have a lot of street that are very narrow but still used by cars (not used like a highway, used when you really need to go there). From a walking Parisian point of view I can tell you that no bus can use these narrow streets but you can be sure there's a bus stop in the next avenue (which is usually close). Thus by using both big avenues (mostly used by cars and with big empty sidewalks) and small streets (full of life, shops and well, some cars) you can achieve a very good "ambiance".

Bikes usually have a small lane on both sides of the road and don't really bother anyone.

I can't recall the name of a famous architect who was working on street dynamics and security but it is basically the opposite of what big names did in cities built for cars like Brasilia, Washington etc... The moral being that if you concentrate people in the streets you will need a much smaller police force because you're always "close" to other people.