but, in general, a hundred dwellings per acre (247 per hectare – high by American standards, but quite common in European and Asian cities) could be considered a minimum. Important terms To understand a discussion about compact cities several key terms need to be defined. The first term is urban density. Urban density refers to how many people live in a square mile of land. It is used as a variable in evaluating how livable a city design is. The optimal urban density for compact cities is high enough to keep residents close to community amenities but low enough to allow residents access to green spaces, reasonable privacy, and acceptable views. Another term associated with the discussion about the compact city commuter is self-sustaining. Self-sustaining means that the city has everything that a person needs within the community. This includes stores, employers, a post office, service providers, energy generation, waste disposal and processing, and small-scale agricultural production (community gardens and/or vertical gardening). Since the objective of the compact city is to make the community as accessible as possible to residents, the term pedestrian design also needs to be defined. Pedestrian design means that the compact city's layout and features support pedestrian traffic. This is an important part of a compact city design because it facilitates the flow of foot and bike traffic through the community. While a pedestrian design will primarily focus on hardscape elements, such as pathways and sidewalks, green space also is important to consider. Green space, defined as the areas of nature found in the landscaping of a community, includes grassy patches, flowerbeds, trees, rock gardens, and water features. Green space is important in compact city designs because they enhance the aesthetics of the community. It also helps to control storm runoff, and they help to remove CO2 and other toxins from the air. Proximity is the final term that needs to be defined. It refers to how close a community's amenities are to where people live. Ideally, compact cities will keep key amenities within walking distance of people's homes. The acceptability of proximity to different amenities is calculated based on travel time and distance. The compact city and related concepts Although the concept of 'compact city' was coined by American writers, it has been used more in recent years by European and particularly British planners and academics. See, for example, the writings of Michael Jenks. In