Is there a Travel-Time Limit to City Growth?

Planners have suggested for 50 years that cities may have an optimum size beyond which they begin to become dysfunctional. However no such size limit has been found based on social, health, economic or environmental factors. The Megacities of the world have continued to grow by becoming more and more dense and continuing to sprawl outward. In recent years some cities like Mexico City have begun to slow down in their growth outwards and those cities that do not densify begin to slow in population growth. This may be explained by the travel time budget. The Marchetti constant on travel time budgets means that when a city grows beyond its ‘one hour wide’ size it will begin to become dysfunctional in transport and human terms. Road rage is just one of these symptoms. The limit depends on a combination of travel speeds and densities. A city with an average speed of 40 kms per hour and 100 people per hectare would become dysfunctional after 12 million; a city of 10 people per hectare and 50 kms per hour average speed will become dysfunctional after 2 million people. Such limits are beginning to be seen and no obvious technological changes are going to change this. Electronic communication is not changing the need for human contact in cities. City limits are one of the driving forces behind the move to build fast rail systems in over 100 US cities and to build rail and bus rapid transit in many developing cities.