The density in cul-de-sac neighborhoods is still too low to make it possible for residents to get to any of the basic services they need (school, grocery, etc) without driving. Adding a few roads to shave a mile or two off a commute or trip the store isn't going to make much of a difference. Our (american) cities desperately need urban growth boundaries to force density.
Why are you/we trying to force density when it's so obvious that most people want the exact opposite? [edit] I had in a house in a great neighborhood in the city within walking distance of grocery, restaurants, buses, etc., and I still moved to the suburbs. Obviously density's not the only thing people want.
The problem is that the playing field isn't level: as Edward Glaeser points out in his book, The Triumph of the City (highly recommended; http://www.amazon.com/Triumph-City-Greatest-Invention-Health...), there are numerous institutional barriers to urban living, including, in no particular order:
1) Mortgage interest tax deduction; since urban life favors multi-unit dwellings but condos have externalities single-family houses don't (it's hard to deal with a noisy upstairs neighbor when you both own), this favors suburbs.
2) Substantial car-based infrastructure investment that means car owners pay around half of the "total" or social cost of their driving.
3) Barriers to entry in urban areas, especially in the form of zoning height limits.
4) Tying education and education funding to geographic location.
I believe there are a couple others I've forgotten.
It's pretty obvious that a lot of people want density—if they didn't, housing prices in NYC, DC, Seattle, San Francisco, and others wouldn't be so high. Hold supply relatively constant while demand increases, and you get prices that zoom up. I would, in many ways, draw the opposite conclusion from yours.