What is the purpose of the transportation system? 

Why do we use transport? Some ways of getting around can have elements of fun to them, like a mountain bike, a jet ski, or (if you’re a nerd, like me) a high-speed train. But in the vast majority of cases, we are not just travelling for the sake of it – we’ve got places to be! 

In a recent article, Greg Shill from the University of Iowa and Jonathan Levine from the University of Michigan examine transportation from these “first principles”. They define the main purpose of transportation as “connecting people to destinations they value”.  

This seems hard to argue with – of course we use transportation to get where we are going – but many of the metrics that are prioritised in transportation planning, engineering, and policy do not directly serve this purpose. Ideally the transportation system would maximise access: how many destinations can be reached with a given amount of money, time, or effort. What tends to instead be prioritised in much of the world is mobility: how quickly people can move to their destination. 

These two goals of access and mobility might seem roughly equivalent. In many cases, that is true. Adding capacity to a congested road might both increase vehicle speeds (mobility), while also bringing more destinations within a reasonable travel time (access).  

But a transportation strategy centered around widening roads and prioritising traffic flow also has the effect of pushing destinations further apart, making walking, cycling, and public transport less attractive and reducing access for those modes. The cost of car ownership, both personal (purchase, fuel, and maintenance costs) and societal (air pollution, congestion, and crashes) become the “price of admission” to the transportation system. 

As an example, Atlanta and Philadelphia are similarly-sized metropolitan areas in the United States, with radically different urban forms and transportation systems. Atlanta is a sprawling city with a comprehensive freeway network that cuts right through the centre of their downtown. By contrast, Philadelphia is an older and much more densely built city, and though it has a freeway network, much of the historic city grid remains intact.   

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Figure 1 Unfettered mobility in Atlanta (left) (https://commons.wikimedia.org/wiki/File:Atlanta_75.85.jpg), 

Superior access in Philadelphia (right) (https://commons.wikimedia.org/wiki/File:Philadelphia_City_Hall_%28146015659%29.jpeg) 

A 2021 comparison of urban access shows that, despite Atlanta’s car-centric development pattern, residents of both cities actually have very similar access to jobs by car within 30 minutes. Less surprisingly, Philadelphians who don’t drive have far more access to employment than their Atlanta counterparts, and are able to reach ten times as many jobs by walking, twice as many by cycling, and five times as many by public transport. 

Transportation planners tend to talk in terms of mobility, sometimes even without realising it. Discussions of intersection level of service, travel delay, and congestion implicitly prioritise mobility. It’s sometimes trickier to visualise or calculate improvements to access, for example by improving pedestrian crossings or building out a connected bike network. Despite this, access is a key concept that should have a big influence on how we plan our cities and transportation system.