There are many "theory" comments below. Sometimes it is best to examine if "theory" of planners (and commenters) is met with actual execution of a "theory". And we have a good example right here in Portland with Portland Development Commission's South Waterfront URA. 

PDC's chief Transportation Planner Matt Brown projected that all trips for South Waterfront would be 40% of other modes than vehicles; walking, tram, streetcar, light rail, biking and bus. South Waterfront provides all these alternatives to the max. But recent studies shows that less than 7% of all trips are by these modes.

 All the planning dictates of South Waterfront were based on this "theory" -it has proven wrong. And similar dictates occurred in The Pearl URA and enormous congestion has resulted.  And the same is being applied to NW Portland and other arterial street density requirements affecting neighborhoods. 

These "theories" need to be questioned with the examples we have here in Portland and other cities.