If you're talking about avoiding car use, the design of developments is far more important than its density. If a neighbourhood is built around a central shopping and services area (which can but does not have to look like a traditional main street) with an adjacent rapid transit station, then transit and walking will have far higher modal shares than a denser neighbourhood built around arterial intersections and highway interchanges.
The traditional Toronto suburban subdivision follows the neighbourhood unit principle. The basic building block is the concession road, which becomes the arterial road following suburban development. At the centre of the neighourhood unit are schools and community centres. Businesses and high-density residences are relegated to the edges, along arterial roads. That's also where we site transit infrastructure. It works reasonably well, in many cases, and it's far more successful than more random patterns of development in American suburbs. The European model is different. Everything, including shopping and other businesses, is at the centre of the neighbourhood, with residential densities decreasing as you move outward. The rapid transit station is also at the centre of the neighbourhood, rather than out along the arterial road. This neighbourhood centre could take the form of a traditional shopping street or of a small shopping centre, but either way it includes the basics like a grocery store and pharmacy. The objective is not for everyone to stay in a self-contained neighbourhood; that's why the transit station is there, so that people can conveniently travel to other neighbourhoods or to the central city for shopping, entertainment, or work.