Advocates and policymakers constantly debate the virtues of different transit modes. Should we build streetcars or BRT? Commuter rail or heavy rail? Each involves technical and cost tradeoffs, but transit advocates often don’t agree. This debate stems from a difference in how people think about transportation. Is the goal to maximize mobility, or accessibility?
Professionals’ precise definitions vary, but in general, mobility refers to the distance or area a person can cover in a period of time. Accessibility is a more qualitative measure about what you can access, not how much ground you can cover. If a given transportation system allows you to easily access your job, a grocery store, and other local retail services within 20-25 minutes of travel time, that site would have good accessibility, even if that 20-25 minute window of time doesn’t allow you to travel very far. Mobility, on the other hand, is transportation for transportation’s sake. It deals only with distances and speeds, and thus, by extension, area covered.
Choosing which concept to focus on affects how land use fits into the debate. Mobility is a pure measure of distance covered, whereas accessibility is more concerned with the ‘what’ than the ‘how far.’ What’s on the land matters a great deal. Increasing mobility usually also increase accessibility: the more area you can cover in a given amount of time, the more uses you can reach. But we can also increase accessibility without actually increasing mobility. In the United States, we have a legacy of designing transportation policy on mobility alone, while ignoring accessibility.
Jarrett Walker observed,
Streetcars that replace bus lines are not a mobility improvement. If you replace a bus with a streetcar on the same route, nobody will be able to get anywhere any faster than they could before. This makes streetcars quite different from most of the other transit investments being discussed today. …
Where a streetcar is faster or more reliable than the bus route it replaced, this is because other improvements were made at the same time — improvements that could just as well have been made for the bus route. These improvements may have been politically packaged as part of the streetcar project, but they were logically independent, so their benefits are not really benefits of the streetcar as compared to the bus.
New streetcars that replace buses do not change mobility. In theory, a streetcar traveling in mixed traffic will have the same mobility as a bus. Jarrett and other bloggers then grapple with mobility versus accessibility and what to measure. Cap’n Transit asks, “Why do we care about mobility?“
Interestingly, Jarrett uses Walk Score to count the places, meaning that his mobility takes density into account. That makes it more valuable than simply measuring how many route-miles you have available to you. There was some back-and-forth in the comments about whether streetcars could increase mobility by increasing density relative to a similar investment in buses, but I don’t think there was a solid conclusion.
Jarrett’s response acknowledges the intrinsic value of accessibility, but also notes the limitations of that concept:
The argument is that the number of places you can get to doesn’t matter so much. What matters is how far you need to go to do the things you need to do. In a denser and better designed city, your need for mobility should decline because more of your life’s needs are closer to you. That’s unquestionably true, and I suspect anyone who has chosen an urban life knows that in their bones. …
One puzzling thing about the access-not-mobility argument is that it suggests that much of what we travel for is generic and interchangeable. Many things are. I insist on living within 300m of a grocery store, dry cleaner, and several other services because I need them all the time and don’t want those trips to generate much movement. But I go to a gym that’s about 1500m away because I really like it, and don’t like the ones that are closer. And every city worth living in is packed with unique businesses and activities and venues that must draw from the whole city. A lot of us want more of that uniqueness, less interchangability, in our cities. How is that possible if citizens aren’t insisting on the freedom to go where they want?
This cuts to the core of the tension between mobility and accessibility. In one sense, increasing mobility naturally increases access simply by opening up easy travel to new areas. However, accessibility captures a more complete picture by asking what the travel is for, not just to accommodate it. Streetcar systems and other rail based transit tend to have higher ridership than similar bus systems. This is known as rail bias, the tendency of passengers to ride trains more often than projected based solely on the mobility improvements of a transit line. Might this rail bias actually represent an accessibility bias?
Metro’s history shows us some of the tension between these two concepts. The Orange line includes areas focused on accessibility between Rosslyn and Ballston, while the outer reaches of the line travel longer distances at higher speeds, prioritizing mobility. Of course, a subway presents an inherent increase in mobility over a bus or streetcar anyway, thanks to the grade separation of the subway tunnels. Still, the hybrid nature of Metro’s system shows the different conceptions of mobility and accessibility.
In Zachary Schrag’s The Great Society Subway, he concludes with a quote from a now retired WMATA official involved with the planning of Metro. Before choosing technologies, routes, and levels of transit service, you have to ask “what kind of city do you want?” One of the key arguments in favor of streetcars is their ability to attract transit oriented development in ways that buses cannot. If we accept Jarrett Walker’s assertion that streetcars do not offer a mobility improvement over buses, what about an accessibility improvement? Transportation investments can be powerful forces for attracting and shaping development, and thus improving accessibility by shaping the city.
In determining what kind of city we want, we also have to recognize that different modes of transportation offer different improvements to both mobility and accessibility. Transit system can accomplish both goals, but design choices inherently emphasize mobility over accessibility or vice versa. Every fantasy transit system makes value judgments about mobility versus accessibility. When those systems are the work of one individual, they represent the preferences of that individual’s vision for the city. How should the Washington region balance mobility and accessibility in future transit and transportation planning?
Cross-posted at Greater Greater Washington.