Mixed-use zoning is making a comeback, and it really fosters walkability. You can have a dense area that isn't that easy to get to things because its zoned too rigidly.

Zoning originally came about for a few reasons. One was that industrial pollution was a serious issue and people wanted that away from other types of land use. The other was the rise of the car. Zoning has allowed places to require minimum lot sizes or state that mixed-use isn't allowed. One of the reasons we have so few walkable communities in the U.S. (even small ones like old towns used to be) is that zoning doesn't permit it. Because of this, areas that are walkable and are mixed-use are extremely expensive to live in.