Commuters, meet Marchetti

https://www.winnipegfreepress.com/opinion/analysis/2024/02/20/commuters-meet-marchetti

IF you’re ever hosting a dinner party and you get the feeling that urban planning theory might be just the right topic to spark some lively conversation, casually bring up Marchetti’s Constant. It might make you the hit of the party.

It’s a simple but powerful idea that has defined the shape and size of cities for thousands of years and is a key determinant of the sprawling metropolises we live in today.

In 1994, Italian physicist Cesare Marchetti published a research paper titled Anthropological Invariants in Travel Behaviour. The study looked at how people of all cultures have moved around their cities from ancient times to the present day and recognized a consistent human behaviour, now known as Marchetti’s Constant. His research identified that when taking the average across a population, the maximum time people will comfortably dedicate to urban commuting is one hour per day — half an hour in each direction.

Marchetti found that the half-hour commute has remained consistent throughout time, and the size of the cities we live in has been defined by how far we can commute in that time, using the transportation technologies of the day.

He looked at Ancient Rome and found that even as it became the largest city on Earth with more than a million people, it never grew past four kilometres in diameter. With an average human walking speed of four kilometers per hour, a person could do a pedestrian commute from the periphery to the city centre in half an hour. Despite incredibly high densities and no natural boundaries to stop outward expansion, the size of the city remained defined only by how far people could comfortably walk.

Cities across the world, from China, to Mesoamerica, to Europe, roughly maintained this size well into the Industrial Revolution.

With the invention of bicycles and the development of streetcars in the late 1800s, the speed of urban mobility increased, and the size of cities followed, but the half-hour commute time remained constant.

Commuters on bikes or riding streetcars could travel about six kilometres in half an hour, and the streetcar cities that developed across North America during this time followed by growing to this dimension. The pre-automobile city of Winnipeg with 400 streetcars serving 100 million riders per year, was about five kilometres from downtown to the periphery, about a half-hour commute.

Today, the size of cities is defined largely by vehicles and the road networks they travel on, but Marchetti’s Constant remains.

Comparing major Canadian cities, average commute times are remarkably consistent despite vast population differences, with the three largest cities averaging about 30 minutes and smaller cities ranging between 23 and 27 minutes. This consistency can be attributed to larger transportation networks in more populous cities. Calgary as an example, covers twice the area of Winnipeg, yet its commute times are only about a minute longer because it has a much larger road network.

Intuitively, this would seem to indicate that cities should be striving to build larger roads to reduce travel times, but this is where the lessons of Marchetti’s Constant become important.

Cities don’t build their transportation networks to create 30-minute commutes, they naturally expand to the 30-minute limit that their transportation systems will accommodate.

We often expect that expanding a road network works like replacing a small pipe with a larger one, creating more room for water to flow smoothly through. When it comes to roads, however, it’s more like installing a larger pipe that naturally draws more water into itself until it’s filled again.

When a road is made larger, people can drive faster, and commute times are reduced. Because of Marchetti’s Constant, people will naturally begin to live farther away, until that magic half hour commute time limit is once again reached. This brings more cars travelling longer distances to the enlarged road, resulting in traffic congestion that increases commute times beyond half an hour.

Residents will again lobby for larger roads in response, and the cycle will begin again. The result is that cities are not only saddled with the prohibitive cost of constructing larger roads, but they must also pay to service and maintain more infrastructure in an ever-expanding urban area. Continually stretched civic budgets, with reduced services and crumbling infrastructure, are the result of cities being caught in this unsustainable growth cycle.