Walkability, density, and mixed-use have become key terms in the conversation about designing our cities to promote healthy lifestyles.  In an interview with behavioral psychologist, Dr. James Sallis of the University of California San Diego in The Globe and Mail,  Sallis discusses how his research reveals key design elements that encourage physical activity.  In the 20th century, the automobile and new ideals in urban planning radically changed the way in which cities were structured.  Residential and commercial areas were divided and highways were built to criss-cross between them.  Suburban sprawl rescued city dwellers from dense urban environments that had gained a reputation for being polluted and dangerous.  In recent decades, planners, policy makers and environmentalists have noted how these seemingly healthy expansions have had an adverse affect on our personal health and the health of our built environment. Today, the conversation is heavily structured around how welcoming density, diversity and physical activity can help ameliorate the negative affects that decades of mid-century planning have had on health. Sallis describes how much of a psychological feat it is to change the adverse habits that have developed over the years and how design, in particular, can help encourage the change.