Source: Adapted from P. Hugill (1995), World Trade since 1431, Baltimore: The Johns Hopkins University Press, p. 213.
There is a relationship between the form and structure of cities, a relationship that was shaped by subsequent changes in transportation technology. One way to express this relationship is through the commuting range, which illustrates a specific space/time relationship. The above figure depicts a theoretical urban form which is the summation of successive transport influences. It assumes uniform friction of distance and capacity of transport infrastructure.
- Walking. Assuming a willingness to commute for one hour, a pedestrian walking at 5 km per hour could cross about 5 km. The space/time relationship of such a commute would be a circle of roughly 10 km in diameter. This helps explain why prior to motorization, cities were compact since most of the mobility took place by walking. The majority of cities developed prior to the industrial revolution have kept a high-density urban core reflective of this constraint.
- Streetcar. A streetcar, like those operating in the first half of the 20th century, could travel around 15 km per hour along fixed lines radiating from the city center. In this case, the space/time relationship would be to reflect the time spent walking to the streetcar line that has to be deducted from the total travel time. Therefore, someone 15 km away from the city center would need to live next to the streetcar line to fall within the range of one hour maximum commuting time.
- Cycling. In the late 19th and early centuries, the bicycle became a mode of mass transportation. With approximately the same speed as a streetcar, but with no fixed line limitations, the space/time relationship of commuting by bicycle would be a circle of 15 km in diameter. The influence of the bicycle on the urban form was tenuous and short-lived. Still, many cities around the world have implemented bicycle-friendly infrastructures such as bike paths and parking areas.
- Driving (no freeways). With a driving speed of about 30 km per hour (taking into account stops, congestion, and parking), an automobile creates a spherical space/time relationship of about 30 km in diameter. This permitted the formation of the first car-oriented suburbs, many of which were serviced by buses.
- Driving (with freeways). Along a freeway, a fixed infrastructure, the driving speed is doubled to 60 km per hour along the main freeway axis. The space/time relationship is, therefore, star-shaped with 60 km of diameter along its axis. However, under such circumstances, a city can become multi-nodal.
This synthetic representation takes a different result depending on historical and geographical factors, including modal preferences. Older cities were more influenced by the prevailing transport technology of the time, while cities that were established more recently would have been influenced to a greater extent by the automobile. For instance, North American and European cities have evolved differently even if they were impacted by the same changes in transport technology.