Can Mass Transit Save the Environment? Right Wing or Left Wing, Here’s a Post Everybody Can Hate

(Photo: RJ Schmidt)

A major rationale — perhaps the major rationale — touted by supporters of mass transit is that by reducing our output of greenhouse gases and other pollutants, transit can help save the environment. The proposition seems intuitive and even obvious: by no longer encasing each traveler in thousands of pounds of difficult-to-move metal, surely transit is more energy-efficient. Plenty of analyses prove this. But then again, Aristotle, who was revered as the infallible font of truth for more than 1,000 years, proved that heavier objects fall faster than lighter ones and that women have fewer teeth than men. Might studies that demonstrate transit is greener be similarly wrong?

They might. The reason is that many studies of energy efficiency by mode often make questionable and — depending on the author’s point of view — self-serving assumptions. The main trick is to look at autos with but one passenger and compare them to transit vehicles in which every seat is full. (For example, see this.)

But in the real world, this is emphatically not the case. At any given time, the average auto has somewhere around 1.6 passengers, and the average (typically 40-seat) bus has only about 10. Rail vehicles typically have more passengers (on average about 25), but then again they are also typically much larger. Thus their average load factor (percentage of seats filled) is also not high, at about 46 percent for heavy rail systems (think subways in major cities) and about 24 percent for light rail (think systems that mostly run at street level).

It is not clear that moving around large and largely empty vehicles is much of an improvement over moving around smaller ones. In fact, it may be worse. According to the Department of Energy’s Transportation Energy Data Book, in 2010 transporting each passenger one mile by car required 3447 BTUs of energy. Transporting each passenger a mile by bus required 4118 BTUs, surprisingly making bus transit less green by this metric. Rail transit admittedly fares better, at 2520 BTUs per passenger mile, but even this is not the kind of slam-dunk advantage over the auto that transit advocates might hope for. 

There are some qualifications here, some of which aid transit and some the car. The sources of transit’s energy—electricity in the case of rail, and often natural gas in the case of bus—are typically cleaner than those of the auto, so that even with higher energy usage, transit may produce fewer greenhouse gases. Still, even taking this into account, a bus produces more CO2 per passenger mile than the car for most trips.

Transit looks better when the environmental costs of building the vehicles and infrastructure are spread over the passenger miles that each mode will accommodate. Taking this into account, an average bus at normal load factors does currently produce slightly fewer pounds of CO2 than autos, and rail transit does much better.

On the other hand, electricity is not much cleaner than gasoline if it is derived primarily from coal, and in some states, such as—surprise—West Virginia, nearly all of it is. And in any event, the electrification of autos is advancing. Moreover, greatly increased fuel economy, even for regular old internal combustion vehicles, is rapidly making cars more efficient. By the time the new fuel economy standards for autos are reached in 2025, a new car’s average mpg will have increased from 27 to about 40, meaning that in terms of energy efficiency, cars may leave buses behind and begin to be competitive with rail. 

Does this mean that efforts to increase ridership will not help—and may even harm—the environment? Not so fast. In fact, the environmental benefits of higher transit ridership might be very great—but they depend completely on how those increases are obtained.

Pumping up ridership by adding transit service will probably do little good, and may even be counterproductive. The reason is that new service promises to reduce transit’s already less-than-spectacular load factors and result in largely empty vehicles.

Why? At this point we have picked all the low-hanging fruit in terms of transit markets. We already have extensive rail transit service in the places where land use and demographic characteristics are congruent with high transit use—such as central Chicago, Boston, the San Francisco Bay Area, Washington D.C., Philadelphia, Los Angeles, and (writ very large) New York City. For the most part, any new transit service has to go to relatively low-density cities and low-density areas within cities, meaning that new investment would drag transit’s overall efficiency down, not up. 

To give an idea of how this phenomenon works, the heavily used New York subway system (58 percent of seats are typically filled) produces .171 pounds of CO2 per passenger mile, less than 1/3 the average for cars nationwide. However, the much more lightly used Cleveland, Pittsburgh, and Memphis light rail systems actually produce considerably more CO2 per passenger mile than cars do. Unfortunately, due to diminishing returns, new rail transit systems are more likely to resemble the latter three than the former one.

On the other hand, if we can persuade travelers to leave their cars and ride existing transit service, rather than new service, the environment will benefit greatly. Given its current low load factors, transit generally has plenty of capacity to absorb new customers with practically zero additional energy expenditure.

Strategies to pursue this would involve economic incentives to influence behavior. These might include pull strategies designed to lure riders onto transit, such as fare cuts, or push strategies designed to get them out of cars onto transit, such as increasing gas taxes, congestion tolling, or charging market rates for street parking. Note that pricing strategies of this sort would also help the environment by raising auto efficiency through increased incentives for carpooling. 

Thus it makes more sense to focus on policies that attempt to change travel behavior as opposed to building infrastructure and buying vehicles. However, public policy in recent decades has self-defeatingly focused on the latter strategy, not the former. This has proven very popular, since shiny new trains and buses allow politicians to take credit for very visible improvements, while the costs of said infrastructure and equipment are essentially invisible since they are spread broadly across the American public and are hidden in arcane budgeting processes. (With a thicket of passenger fares, fuel taxes, sales taxes, bonding, advertising revenue, etc. coming from federal, state, regional, county, transit district, and municipal administrations, it can be difficult even for experts to determine who pays what for transportation.)

So there it is: to benefit the environment, probably the best thing to do is be very skeptical about adding new transit service and even to discontinue some service we are currently providing (sorry, liberals). Simultaneously, we should raise fees and taxes for driving (apologies to you conservatives). The best I can offer to keep the comments section below free of hate posts is that at the moment the chance of either happening is small. This may make good sense given the current political atmosphere—but it makes little sense for the atmosphere of the planet.

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  1. joe says:

    This all assumes that people come first, and then mass transit comes later. What if we acknowledge that density of cities is endogenous (aka, density can be CAUSED by mass transit, and less density is caused by cars). If we acknowledge this factor, then your argument of diminishing returns doesn’t hold (along with most of the other calculations). Why do you think Houston is less dense than New York City? Because one had mass transit, and was built around it, and the other did not and it was built around the car.

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    • Praxis says:

      The pattern of diminishing returns from transit expansion has been demonstrated empirically. It just doesn’t attract enough people.

      Houston is less dense than New York City because it was built in the automobile age. New York is dense because it was built before the mass affordability of cars. Travel speeds were much slower, so buildings had to be closer together. There’ll never be another New York. Almost all new urban development will be low-density and car-oriented.

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  2. Bob F says:

    See the following article that came out a week before this one:

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  3. Bob F says:

    The energy benefit of transit is it allows you to build compact cities and still get people where they need to go. That has the effect of reducing travel needs for everyone, including the 80% who will still drive to work. As we build out new “Smart Growth” communities, we can build in a way to minimize travel distances — and then add transit service to the extent that congestion and demand warrants it.

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    • Praxis says:

      We could build even denser cities and walk or bike everywhere. That would save even more energy. But people don’t want to live that way. And they don’t want to live at transit-oriented densities either. Cars are just a much faster, more comfortable, more convenient and more practical way of getting around. That’s why they’ve displaced transit almost completely, and why for 50 years we’ve been designing our urban areas mostly for cars instead of buses and trains. The only urban areas where transit still has a substantial share of the transportation market are old dense cities like New York that were built before cars became widely available, and where driving (and parking) is difficult and expensive because of the lack of space.

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      • Gerry says:

        You are ignoring new development in communities like Portland, Minneapolis, and Denver. And how many people are jumping to move to these places versus Houston and Detroit? As a redisent of a small town forced to drive into the suburbs due to my job, I am very sick of people saying that this is some kind of choice that I am making.

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      • Praxis says:

        Most new development is low-density and car-oriented, not high-density and transit-oriented. That includes most new development in Portland, Minneapolis and Denver. The most recent Census data shows that the vast majority of growth in the U.S. is occurring in suburbs, not in dense urban cores or downtowns.

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      • Green Mountain Bot says:

        Minneapolis has had the largest total growth of any city in Minnesota over the last 10+ years, and Saint Paul is second. Some cities have had higher percentage growth, but in terms of bodies moving in, the core cities are ahead of the rest.

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  4. Frank says:

    It is tremendously refreshing to see an honest analysis of the relative costs and environmental impacts of public transit and private cars. Perhaps the evaluation can, in the future, be expanded to include the cost (value) of riders’ time spent traveling. I haven’t read other comments on this piece yet, but I can be sure it is filled with virulent anti-car rants filled with the transit fanatic’s trademark refusal to accept the facts.

    Thank you to Freakonomics for once again injecting fact into the discussion.

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  5. Chris Theis says:

    This story brings up one of my pet peeves, HOV lanes. In my narrow view of driving the streets of Denver it seems that HOV lanes create more congestion than not. I don’t think many drivers are changing their driving habits to take advantage of the HOV lanes, so the overall effect of the HOV lanes is more pollution.
    As I have sat in the traffic jams and watch the empty HOV lanes I have wondered if Freakonomics has had the audacity to disprove the efficiency of HOV lanes. I am sure politician, left or right will ever take this on.

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  6. Jei says:

    Your biggest mistake is that you assume the only place to add new transit is in low-density areas. With crush-loaded transit vehicles during rush hour, adding more vehicles easily increases ridership while remaining extremely efficient. Interestingly, mass transit probably follows the same rules as highway capacity–in certain markets, you can never build too much. Think about it, would you switch if the vehicles came every 3 minutes during peak? Wouldn’t everyone?

    The best incentives to ride transit is not actually the cost of driving vs. riding, but the time penalty. Transit has to take no more than 10-15% longer. Investments to make our transit faster than driving in markets where that is possible is also a good investment.

    Also, on an individual level, your choice to ride transit is always a carbon benefit. The marginal carbon cost of an additional passenger is very low.

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  7. Gerry says:

    The big factor not mentioned here is the development patterns that transit encourages. Consider Portland, OR, where dense development, with a small footprint, leads to lesser impact on the environment. Compare that to places like Houston and Detroit, developed around the automobile, where long commutes emit large amounts of pollutants and encourage land use that destroys the natural environment.

    As an electrical engineer, I take strong issue with the idea that electrification of automobiles is around the corner. While GM and Nissan have accomplished wonderful things with the Volt and Leaf, there is no sign that the economics of these vehicles will ever reach a point where a middle class family could afford two of them. Engineers do not magically invent more fuel efficient cars whenever policy makers snap their fingers, so the mandated increase in fuel economy standards is by no means a guarantee of tomorrow’s reality.

    Development patterns in these low carbon footprint cities- like New York, Chicago, and San Francisco, depend on transit. These cities simply would not exist as we know them without transit.

    Electrified transit is the proven technology that we have right now to drastically clean up how me move ourselves and develop communities. California’s bullet train, for example, is planned to operate with 100% renewable energy. We need to break the cycle of addiction, reject the temptation of questionable promises, and improve our country with what is already there for us- electrified transit.

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  8. Gary Hewitt says:

    There are three ways that public transportation can reduce carbon emissions:

    1) Reducing Auto Trips: This is only variable considered in your blog post.

    2) Reducing Congestion: Auto benefit from reduced congestion when public transportation is available. The reduced idling and travel time for autos reduce emissions.

    3) Allowing for Compact Development: The existence of public transportation allows compact development where walking and biking are possible. These reduced trips also have a positive impact on emissions.

    The American Public Transportation Association has a more thoughtful way of calculating the Green House Gas Emissions for public transportation. It is available at

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