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.


Hm, one thing I'm wondering is, by taking a car off the road when converting a driver to a commuter, does that reduce congestion in the roads, thus leading to an increase in gas mileage for drivers? If that's a large enough effect, should the lowering of car pollution due to the higher MPG be credited to public transit, not to cars?


Reducing congestion also incentives more people to drive

Michael Peters

But no more than before. Capacity hasn't changed you've just reduced congestion so it's still a net win for everyone.

Laurent Duval

Are averages really pertinent when discussing mass transit load ratios ? I mean, if you're taking a subway ride at rush hour on a very busy line to commute between your home and your workplace, you won't suddenly switch to that very quiet line over there, far from both, just because there's an incentive to use mass transit more. Of course, adding more overloaded buses, subway cars and trams on busy lines will raise the average, but at the probable cost of more users opting out due to discomfort (or even insecurity, as it's the case in my city, Paris).


Here in Wales ,one gets a free bus pass at 60 years old- this is the best incentive for me to use the bus and according to your article I'm only filling an otherwise empty seat- my partner and I only need one car between us ,the car gets used less so lasts longer and congestion and CO2 are reduced. I must be in eco-heaven already -and if you get on the double decker the view from upstairs of the hills one side and the sea the other is stunning.


The answer is autonomous cars. Distributed, flexible, & minimal infrastructure costs.


You might also consider that having a car also encourages people to travel longer distances than they typically would just riding mass transit. So even if the passenger mile efficiency is worse it could still lower emissions overall due to fewer miles traveled.


I see a lot of sides to this debate and one is on the price component, specifically those that argue it's too expensive of a project to take on...but do the detractors pay attention to the cost of car ownership?

Wow...$8,946 for the average sedan and $11,370 for the average SUV...ANNUALLY. Now multiply that out for the number of vehicles on the road (there are stats for that, too)...

So let's take the lesser figure ($8,946) and reduce the total number of cars by just 1% (2.5 million cars)...and redirect those funds. Hey, we just came up with $22.3 billion.

Yes, it's much more complicated than that...just pointing out the argument of "I don't want to pay billions for my transportation" is seriously jaded as we already pay trillions annually to this for our cars. If we could convert auto manufacturing plants quickly to create tanks for the military in the 1940's, surely we could convert empty factories (or live ones, for that matter) to create parts necessary for rail in the 2010's, right?



I'm surprised that in your final paragraphs you didn't talk about the central factor that makes mass transit work - population density! If you can't build high density housing near your mass transit stations, then yes it is pointless, but if you can then it clearly seems worth it.

Enter your name...

I didn't think that greenhouse gas emissions were really a big goal for promoting mass transit. After all, we started promoting mass transit even before anybody knew what a greenhouse gas was. I thought the main goal was to get more people to/from work during rush hour (in particular) without having to waste real estate and infrastructure on places to drive and to park quite so many cars.


In the US, especially here in CA, the CO2 argument has been used a lot recently. Basically, whatever argument the proponents think is currently the trendiest is used. It has been about the environment, creating jobs, helping traffic, and myriad other half-truths.


Seems to miss a couple of points. First, if you have your existing electric-powered transit system running on coal-fired electricity, it is pretty easy to switch that system over to electricity generated from natural gas, nuclear, wind, or dilithium crystals. Not much you can do with petrol-powered cars if petrol becomes scarce.

Second, I think the real benefit comes from forgetting about transit altogether. This is the 21st century: if I want something - this post, say - to be seen by perhaps thousands of people all over the world, I don't have to hand-write a copy for each one, and send out messengers on horseback or sailing ship to carry the copies to their destinations. I just send out a few electrons & photons (fiber-optic cables) instead.

Much work, and in particular much of the work being done by mass-transit commuters living/working in congested urban areas, could similarly be done simply by sending out those lightweight, energy-efficient electrons & photons. Why not a comparison of that to other transit systems?



Bingo. The key is less transit, not mass transit.

A lot of our traffic problems would simply disappear if we could break away from the traditional office. Telecommuting is just one (although probably the best) possibility. Even in jobs where a physical presence is absolutely required, businesses could do things like extend work hours outside of the traditional 9-5 so that workers can drive or ride in outside of rush hour. Large companies might locate small satellite offices near dense population centers, rather than having a single mega-location in a central commercial district. There a tons of possibilities but little incentive for businesses to change how they work.

I think that even conservatives (like myself) would be open to price incentives. The trick to winning them over is to make new taxes and cut old ones. It's hard to object to a revenue neutral tax.

Tim Johnson

This might be bad for liberals and conservatives, but good for libertarians. Decrease public spending on unnecessary items while raising consumption taxes? Sign me up.


In order for any mode of transit to gain ridership it must provide service from where people are to where they want to go. In the decades since the hayday of municipal rail and bus, people have moved from concentrated multi unit apartments to single family houses in the suburbs and beyond. Similarly, jobs have moved from large factories and office buildings with thousands of employees "downtown" to smaller facilities in office and commercial parks -- once again in the suburbs and beyond.

It is unlikely that central planners in the US can reverse this trend in any time frame shorter than decades. And if they do, what happens to all the abandoned real estate?


I live in Calgary, Alberta (Canada) and our public transit system consists of both light rail and buses. During morning and afternoon rush hours, the entire system is oversubscribed. Stories are common about people waiting on a cold morning for a bus and unable to get onto it when it arrives because the bus is already full to bursting, and so the commuters must wait for another one. The trains are jam packed.

The point is this: during the middle of the day once everyone is already AT work or school, yes of course buses and trains run at lowr occupancy. This is to be expected. For the very same reason there are fewer cars on the streets at 2 PM then at 8 AM. This should be factored into calculations... How much less energy per capita is a jam packed train using than all those vehicles stuck idling in a traffic jam during commuter peak times?

Secondly, notwithstanding the above, I actually think that the majority of the users of the system have their own incentives to use mass transit that have nothing to do with CO2 emissions. Some of those reasons include the very high expense of parking in our city (which is amongst the highest in North America), lack of available parking at colleges and universities, the desire to avoid heavy traffic, people who don't own cars, and so on.

Now, the net result of so many people using mass transit during rush hour is that there are indeed fewer cars driving around, which means less pollution, but I would strongly argue that's really just a handy side-effect, a positive externality, to providing people with the means to move around without driving themselves.



Interesting points and a worthwhile read.

Not directly related to the environment, but what about productivity losses due to commuting or driving? Where I live, and I imagine this is true in many other areas, mass transit doesn't save me time. For some, I'm sure mass transit avoids being stuck in traffic and actually saves time and not just hassle, gas, CO2, etc.

Joel Upchurch

Brad Templeton has been engaged in these issue for years. Read this on his blog.
He does a lot of analysis on robocars and robotaxis. I'm interested in robobuses, where people can connect into a website where say their stop and the destination and time. The system then texts you your pickup time. The buses won't run when there is little demand. The current system where people drive the buses and the drivers keep driving even when there is no demand is very inefficient.

The problem with current cars is that the cars weight too much and they haul too much dead weight around. If the average vehicle weighed a few hundred pounds, then the BTU per passenger mile would skyrocket.

John Halunen

Pretty sure bicycles blow the doors off the efficiency of mass transit even in NYC? Would help with our obesity problem too.


It seems like best solution is to start building cities to look more like New York, dense urban centers are more efficient in almost every way and are generally the biggest wealth producers in the world.