5 miles is about the largest commuter zone for foot traffic. Without cars, small towns are often spaced 10 miles apart. Without taking other factors into account, we could imagine these small towns forming a grid in every direction, which roughly 10 miles between each small town, allowing people to walk no more than 5 miles to the nearest town.

Businesses that provide specialized goods and services that are utilized less frequently, and/or that cost more, can only be supported by a larger population. These businesses therefore congregate in larger towns, and ultimately, in cities, where people will travel for less frequent, more substantial purchases.

Proximity to oceans and rivers provides the cheapest, most efficient means of transportation, especially of goods.

Proximity to natural resources can be used for the sustenance of the city.

Proximity to mountains can make transport and trade more difficult, but can afford a city more protection and natural resources.

• 1091: Freiburg founded; over the next several centuries, the city becomes home to a cathedral and a university, which remain landmarks

• late 1800s: Electric trams implemented

• 1948: Freiburg rebuilds along original medieval street plans

• 1955: First land-use plan expands city limits at suburban densities and favors automobiles

• 1970s: Anti-nuclear energy protests at nearby Wyhl spurs the conceptualization of Freiburg as a "Green City"; Freiburg decides to maintain and extend pedestrian zones, bike paths, and light rail

• 1980s: Expansion of light rail and introduction of new, consolidated public transportation ticket

• 1990s: Freiburg integrates urban and transportation planning as a "city of short distances", and further integrates its regional railway system; new development within the city favors public transit and pedestrian commuters over automobile access and traffic

• 2000s: Further extension of light rails and cycling paths

• 2010s: New forms of mobility management, especially through e-mobility; implementation of low-emission zone and noise reduction plan

• 1688: A ferry between Philadelphia and what will become Camden is the first public transportation in the region

• 1831: First rail lines and privately-owned stage coaches are the first forms of mass transit in and around the city; before steam engines, rail cars are drawn by horses

• 1850s: Streetcars appear and expand through the next decades

• 1890s: Trolleys replace horse-drawn street cars and briefly implemented cable cars

• Early 1900s: Lower fares, combined with city government regulations promoting the working class's ability to live further from work leades to widespread use of public transportation

• 1920s: buses replace street cars

• 1902: Three private companies run streetcars and railway systems between the Jersey Shore and Harrisburg

• 1950s: Subway, bus, trolley, and commuter rail are run by five privately owned companies operating in the greater Philadelphia region, and lose ridership after WWII as a result of the rise in car ownership

• 1963: The Pennsylvania General Assembly creates SEPTA (Southeastern Pennsylvania Transportation Authority), which consolidates the subway, bus, trolley, and commuter rail lines in Philadelphia and four surrounding counties

• 2010s: SEPTA launches its sustainability program, a series of 5 year plans with three pillars of sustainability: Natural Environment, Healthy Commuters and Workforce, and Economic Vitality