The following question was recently posted on Quora:
“Is public transportation a real solution to reduce air pollution in big cities?”
Learn what experts in the field believe:
Francis Chen, urban planner:
Air pollution is a consequence of massive resource consumption, and there are different sources of air pollution, not just cars.
However, I’m going to speak from a North American perspective (Hong Kong, as Andrew Leyden points out, has different sources of air pollution, given 90% of people take transit there already).
I argue that land use, not public transit, plays a bigger factor in reducing greenhouse gas emissions. Without proper land use, you can have lots of buses go any where, but they wouldn’t be able to meet the right ridership since everything is far apart.
As Seth Andrzejewski points out, land use is very critical. Your goal: you want to be able to reduce the travel time from your origin to your place of destination, so you choose the mode of transportation that is the least costly, from a time and monetary perspective. You want to reduce the time it takes to take to get to work or buy groceries.
For example, if you can make the places between where you live and the places you want to go (grocery store, work, park, etc.) closer to each other, you make it a lot easier to depend on the car, since things are not spread out. If things are too spread out, the Golden Triangle of Transit here fails:
From Jarrett Walker, international transit consultant: (Human Transit: basics: conceptual triangles).
In short, without the right density and walk-ability, transit services can’t justify frequent services (buses coming every five minutes!) or have the right ridership to meet the service. This is why in a lot of suburbs, you have buses coming every 30 minutes or hour: it is much more cheaper for them to take that route because not many people take transit.
Without proper land use planning (which is a whole other subject on its own, as the comments on Seth’s post point out), like in the US, you have a lot of suburbs, which makes it more convenient to drive than to take transit.
Jarrett Walker, from his book, Human Transit: How clearer thinking about public transit can enrich our communities and our lives (see here: human transit (the book): introduction ) notes that if land use (“development”) is spread out, transit would not be very efficient:
The red line is the transit line, the dark purples are the first and last stops, and the light purple are the intermediate stops.
If we pretended, this was a suburb, having cul-de sacs (in light purple) would not be very efficient for transit vehicles because the bus/rail would have to go through every destination. It would take a really long time for that red line to go to every place.
This is a different transit line, but much more straight forward.
Now, with the proper land use that is very dense in one line, this can be a very efficient transit line. This is key: proper land use. You can still have a straight line but it can be located in the suburbs, and it would be an inefficient transit line because there is not enough ridership
Here’s an example of density, visualized, from the architecture firm, Perkins & Will, of a urban design plan for Vancouver, Canada, I believe.
Here’s another similar image of Vancouver, probably at a different location:
Watch that transit line pass through that dense neighborhood! With the amount of development, it will have a justifiable amount of ridership to justify frequent service along the line to take transit!
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So, in short, land use, not public transit, in North America, has a greater factor in influencing greenhouse gas emissions.
Note: you might have noticed that I was sort of repetitive in different ways. It is really important to be clear when discussing or talking about transportation and the language of transportation because people interpret things different ways. That’s why I provided lots of different visuals from different sources to help put everything in context.
The concept of density, for example, is a very broad concept. Does it mean 5 story, 10 story, 80 story (Manhattan), etc. There are different ways of measuring high density. Similarly, the concept of mixed land use or closer land uses is also very hard to visualize and thus has to be explained in depth to the reader.
This is the role of urban planners, transportation engineers, and also to a growing extent, the smart cities / big data movement, which I talk more about in this article: http://www.quora.com/Smart-Cities/Who-are-the-leading-experts-in-the-world-on-smart-cities-and-data-driven-urban-planning/answer/Francis-Chen-2
Seth Andrzejewski, transportation planner:
Yes, public transport could theoretically be a major solution to air pollution but keep two things in mind:
1. There are many other sources of air pollution, not just from autos. I couldn’t immediately find a % breakdown of where air pollution comes from, mainly because it’s an apples to oranges comparison. Factories or refineries emit different pollutants than autos.
2. The “public transport” solution shouldn’t necessarily be viewed as a public transport problem. The real problem is how many American cities are organized - where people live, work, and shop. Take “auto-centric” Los Angeles for example. It does have a metro, metrorapid bus, and light rail system, mostly serving downtown. For commuting patterns between these points (i.e. from Hollywood to Downtown along the Red Line), there may be a significant transit mode share (say >20%). However, public transport would have to bridge a huge number of possible origin-destination combinations to make a serious dent in auto generated air pollution. People live in Torrence and work to Pasadena, live in Long Beach and work in Santa Monica. It would be much less expensive if we could convince people to live nearer to where they worked.
Andrew Leyden, app developer:
It’s not a slam dunk, as I write this answer from Hong Kong where something like 90% of the people use public transit daily and the air pollution ranks amongst some of the worst in the world.
Pollution isn’t just cars. Here for example we have coal fired power plants and the downwind effects of being located near “China, Inc” and the factories on the mainland. And on top of that, we also have problems with--get this--public transit. The buses they use for public transportation here are very dated and pump considerable amounts of pollution into the air. Efforts to upgrade to the new Euro IV standards have been stalled by concerns about the cost of new buses, etc.
So while it might help in some places, the problem with air pollution is that there is no one source causing the pollution and thus, there is no one answer or cure. You need to look at air pollution in a more “whole” sense--examining all the potential impacts and then act in a coordinated manner to reduce the pollution.