This article from Kirkland, Washington describes in detail an interesting scoring scheme they applied to all of their neighborhoods. They have a good run-down on why walkable neighborhoods are good.

The ability to retain, create, and enhance 10 minute neighborhoods has benefits for users of the neighborhood and benefits for the community as a whole.

• Health. Residents who walk or bike regularly are healthier and therefore walkable communities make it easier to live healthy lifestyles.  According to the Centers for Disease Control and Prevention, people living in walkable neighborhoods get about 35 to 45 more minutes of moderate-intensity physical activity per week and are substantially less likely to be overweight or obese than people of similar socioeconomic status living in neighborhoods that are not walkable

• Traffic. Residents with convenient access to local goods and services are less likely to drive.  If they do drive, they have a shorter travel distance.  The 10 minute neighborhood acknowledges the value to Kirkland’s transportation system of every trip not taken and every mile not driven.

• Transit.  Better access to transit equates to more transit users.  Regional data show that people who live within a half mile of a transit node commute less often by single-occupant vehicle (SOV) with a higher percentage using transit, carpooling, and walking or bicycling to work .

• Demographics.  21 percent of the population aged 65 and older does not drive – and that segment of the population is projected to grow significantly .  Older non drivers need options so they remain engaged with their communities.

• Clean Air.  Less traffic means cleaner air and less greenhouse gas emissions.

• Social Connectivity.  Pedestrian activity and local gathering places help build social cohesion and eyes on the street help people feel safer in their communities.

• Market Forces.  Recent surveys indicate that a majority of Americans want to live in walkable neighborhoods served by good transit .  Those numbers are significantly stronger for younger Americans and those who plan to move in the future, a strong representation of the future real estate market.

• Stronger Retail.  A local customer base is good for local businesses.