North America the term 'smart growth' has become increasingly common linked to the concept of 'smart city'. The concept of 'smart growth' is very similar to 'compact city', although 'smart growth' carries more strongly normative connotations, implicitly accepting the emphasis in current mainstream debates that growth is necessary and good. The term is often used loosely to accommodate these debates Cognate concepts include 'sustainable urban development' with no presumption that development equals growth. Another alternative concept is 'green urbanism'. Steffen Lehmann has extensively written about the compact city and green urbanism. His work presents a series of international case studies and outlines 15 core principles for the design of compact, sustainable cities. The compact city and proximity Compact cities are intended to provide all one needs to live in one community, including work opportunities. Someone who works in a compact city can walk or bike a short distance to work instead of driving. This reduces fossil fuel consumption and emissions and pollutants, as well as traffic density. This is a main selling point of a compact city. However, not everyone finds work in the city, and consequently many people commute to neighboring cities for work. This has two drawbacks: commuting time and the impact of commuting. A study by Boussauw et al. found that commute times depend on urban density and the diversity of structures found in the community. The findings show that shorter commutes correlate with high values for density, diversity, and job accessibility. Thus proximity to work within a compact city needs the city to include structures that are appropriate for businesses in different industries; it also depends on the city's density of development. The more developed the city is, therefore, the more employers can be located there, and the fewer employees will need to commute long distances. Proximity also affects residents' access to green spaces. This is related because the more green space a community has the lower the development's urban density, thus increasing average commuting distance. In a study by Wolsink, participants viewed community proximity to green spaces as important, because green spaces provided recreational and educational opportunities, enhanced mental health, made the city more attractive. These benefits made additional time in commutes acceptable to residents. However, to remain acceptable, the proximity range needed to remain reasonable. Compact cities and sustainability Another selling point of compact cities is that they are supposed to be sustainable developments.