It is true that urban runoff supplies toxic pollutants to creeks and rivers and the air. Furthermore, such patterns consume farmland, forest and wetland. Farms and forest are naturally beneficial. The former, if well managed, can produce less toxins than residential lawns, and can always revert to forest and wetland in the future. It also allows for efficient technical solutions for runoff, such as improved sewage systems.
More density means more crime, and congested roads.
According to Portland, Oregon's chief of police, community policing is made much easier with designs that include pedestrian-friendly streets, a diversity of housing types and densities, and a diversity of people of different ages, incomes and cultures who get to know one another. Driving around spread-out suburban areas is very inefficient for police patrols, and many such areas never see any police presence. In fact, high concentrations of population with mass transit alternatives have far fewer cars then sprawling communities. One recent study showed that a doubling of density reduces the total number of vehicle miles driven in an area by 20 to 30 percent. In contrast, car ownership patterns and driving habits in the suburbs dramatically increase the number of miles driven and trips taken.