Transit energy efficiency

Transportation, including cars and trucks as well as transit, accounts for over 40% of greenhouse gas emissions in Massachusetts.  Greenhouse gas emissions from transportation have risen since 1990, while other parts of the economy — electricity consumption and building energy use — have made progress in reducing emissions.. See the Massachusetts greenhouse gas inventory.

On any given day, choosing to take the T instead of driving is an important way to reduce carbon footprint.  That bus or train is going to run with or without you, so, at the margin, taking the T is carbon free.   And, of course, keeping your car off the road reduces congestion for others.

But, from a systemic standpoint, we need to do more than expand transit use.  If we are to achieve our greenhouse reduction goals, we need to make ridership-driven decisions about what transit services to expand, we need to electrify more of the transit system and we need to accelerate the transition to renewable energy sources for our electricity.

The systemic efficiency of transit is a complicated question that turns on several factors:

  • The average number of passengers per vehicle, not just at rush hour, but all around the cycle.
  • The energy consumption of loaded vehicles per mile traveled.
  • For electrified transit vehicles — the subways and trolley buses — the power generation sources for the electrical grid.

From a carbon perspective, a lightly-used transit route can be a negative. Diesel buses get only a few miles per gallon.  If they have fewer than roughly 10 riders, diesel buses emit more per passenger than single occupancy vehicles.  Using our current fossil sources of electricity, trolley buses aren’t much better.

Although bus riders on every route of the MBTA system often experience over-crowded conditions at rush hour, overall average ridership of MBTA buses runs at about 13 people.  Computations of average ridership on a route have to include return trips and off-hour trips. A bus may be full during the morning rush hour as it runs into town, but may be lightly occupied as it cycles out for its next trip in. Additionally, to provide commuters adequate flexibility, buses need to run into the evening when ridership declines.

The primary source for the computations below is the National Transit Database (NTD), a federal resource to which major transit agencies, including the MBTA, report monthly. The table below shows computations made directly from the NTD and compared to the Oak Ridge National Laboratory’s Transportation Energy Data Book (TEDB) (online tables). Computations and additional details are presented in this spreadsheet.

Energy and Carbon Efficiency of MBTA Transit Modes

MBTA SubsystemBTU/
passenger mile, 2016
Pounds CO2/ 1000 passenger miles, 2016NOTE: See this spreadsheet for computations. Note that BTU/ppm is based on end-use energy, while CO2/ppm factors in generation and transmission loss.
Light Rail (Green Line)1,248325BTU/ppm checks to TEDB Figure 2.07, Energy Intensity of Light Rail Transit Systems, 2016. Carbon emissions based on 2017 in-state generation efficiency, adjusted for line losses.
Heavy Rail (Red, Orange, Blue LIne)1,169305BTU/ppm checks to TEDB Figure 2.08, Energy Intensity of Heavy Rail Systems, 2016. Carbon emissions based on 2017 in-state generation efficiency, adjusted for line losses.
Commuter Rail (Purple Line)2,525409BTU/ppm checks to TEDB Figure 2.09, Energy Intensity of Commuter Rail Systems, 2016. Carbon emissions based on diesel fuel emissions factors (22.4 lbs CO2 /gal).
Motor Buses4,057580Average ridership is 13 passengers. TEDB shows 4,102 as National Average BTU/ppm assuming 9.1 passengers on the bus and diesel fuel mileage of 3.7mpg: Table 2.14 Energy Intensities of Highway Passenger Modes, 1970–2016
Trolley Buses2,008523No TEDB comparison. Carbon emissions based on 2017 in-state generation efficiency, adjusted for line losses.
Single Occupancy Vehicle at 25 mpg5000784TEDB shows 2,939 BTU/ppm, national average assuming 1.5 passengers and gas mileage of 27.6 mpg: Table 2.14 Energy Intensities of Highway Passenger Modes, 1970–2016. Using a lower mpg and assuming SOV may be more realistic for urban commute comparisons.
The Ride15,0102,353No TEDB comparison. This low efficiency may be inaccurate or may reflect that energy consumed includes unoccupied “dead head” pickup trips while passenger miles only count while the vehicle is occupied.
Ferry11,7601,903No TEDB comparison.

While the gap between cars and transit might not be as much as one would expect, the numbers are consistent with other results. For the rail modes, national data allow comparisons to other systems. The MBTA comes out in the middle of the pack. For buses, the MBTA’s energy efficiency is close to national average values. A Better City produced an emissions calulator which computes similar values to those computed here.

Comparisons across modes should be made with caution. Different modes solve different kinds of problems and confront different conditions. Inner city buses, for example, engage in heavy stop-and-go traffic, while commuter rail moves uninterrupted between stations. Different routes within a given mode will likely look quite different from an efficiency standpoint.

The main conclusion from this analysis is that, as we seek to expand and improve transit services, we need to keep ridership in mind. Of course, this is an economic necessity, but it also makes environmental sense — mass transit vehicles are large and require a lot of energy for propulsion.

An additional conclusion is that to achieve greenhouse emission reduction goals as well as air quality goals, we need to push for fuller electrification of the transit system as well as greener electrical power sources.

The MBTA is moving forward on pilot testing of battery buses that would give us the flexibility that trolleys lack to move around construction obstacles and to pass each other so as to provide express service.

The $10 billion question is how fast we can move towards electrifying our commuter rail system, which is currently powered by diesel locomotives.  The MBTA and MassDOT boards will be reviewing high level options for improving rail over the next few months, as the Regional Rail Vision Study delivers its final results.  The legislature may weigh in on those options as we shape the transportation bond bill that the Governor has filed.

Electrifying private vehicles

Several readers pointed out that the potential for green house gas reduction through public transportation electrification is limited. Most of transportation emissions come from cars and light duty trucks, so the benefits of electrified transit systems only take us so far. 

According to data reported to the Federal Highway Administration, There are approximately 5 million cars and trucks registered in Massachusetts and they travel approximately 12,000 miles per year for a total of 60 billion vehicle-miles traveled.   Since there is an average of more than one person in each vehicle, the total number of passenger-miles traveled is something higher than the 60 billion vehicle-miles traveled.   By contrast, all of the buses and trains of the MBTA system account for under 2 billion passenger-miles traveled.

Under the most ambitious scenarios for transit improvement, ridership might double on a few lines over the next twenty years.  But I have seen no scenario under which overall transit trips double and even a transit success of that magnitude over the next 20 or 30 years would still leave the vast majority of passenger-miles-traveled outside the transit system.

Similarly, from a fuel consumption standpoint, all the non-electrified vehicles and trains of the MBTA system account annually for only 25 million gallons of diesel oil and gasoline (or for natural gas, gasoline gallon equivalents).  By contrast, total motor fuel use for all vehicles is over 3 billion gallons annually in Massachusetts.

Somewhat arbitrarily, we have set 2050 as a target year by which we should have dramatically reduced carbon emissions.  I am personally convinced that that target, while hard to achieve, is nowhere near aggressive enough, but let’s accept that target for the sake of argument.  Even given that long lead time, the small current size of our transit system and the limited plausible rates of its expansion mean that electrifying cars and trucks will be much more important from a carbon emission reduction standpoint.

A planning analysis from the City of Boston indicates that electrification of light duty vehicles is relatively efficient as a strategy for reducing emissions. (See Carbon Free Boston, page 65, Figure 29.) The City’s analysis is based on “back of the envelope” computations, but appears to plausibly rank order the options.

Additional Notes on Travel Mode Split

The National Transit Database, referred to above, shows a total of approximately annual 400,000,000 unlinked transit trips on all transit agencies in the state. That would imply a somewhat smaller number of “linked” transit trips. A trip involving a transfer would count as two unlinked trips, but one linked trip.

A higher estimate of transit trips comes out of the 2011 MassDOT Travel survey. Table 66 of that document groups people by the number of transit trips they make in a week. That table implies a total number of annual transit trips a little over 600,000,000. The table does not make clear whether those are unlinked or linked transit trips, but the most reasonable interpretation is that they are linked trips. The survey involves self-reports, sampling and complex statistical weighting, while the NTD is based on more direct measurement, so the NTD is likely closer to reality.

Table 89 of the travel survey shows 68.1% of workers driving themselves to work and 12.6% using bus/public transit. 6.5% worked from home; 4.8% walked; 1.7% biked; 3.8% were passengers in an auto/van/truck. But Table 77 shows that Work/Job accounts for only 8% of trips. If all trips, including non-work trips are included, walking goes up to 19.0% and transit (bus/train) goes down to 7.6% and more driving trips are shared. The survey does not include distances.

Table 31 of the travel survey counts households by number of vehicles and implies a total number of household vehicles of approximately 4,100,000. This comes in a little under the Massachusetts Vehicle Census, which is available online from the MAPC. The MVC relies on registration data and counted 4,593,128 total registered vehicles and 4,235,479 registered passenger vehicles in the state as of the first quarter of 2011, which corresponds to the period of the travel survey.

Unlike the travel survey, the MVC does include distances traveled based on vehicle inspection odometer reads. In the last quarter of the MVC (2014:Q4) there were 46.4b passenger-vehicle miles traveled (annual rate). Including commercial vehicles, the total annual VMT rate was 50.0 billion — not too different from 2015 FHA statistical abstract cited above which reports 59.2 billion. The abstract series is not available past 2015, but the 2017 FHA number is 62.7 billion VMT for Massachusetts. The FHA data derives from state reported data which appears to be based on a mix of modeling techniques some of which may involve traffic counts and road lengths as opposed to vehicle odometer mileages. The FHA data may include miles traveled on Massachusetts roads by non-Massachusetts vehicles.

Summarizing: The transit share of passenger miles traveled in Massachusetts is hard to precisely estimate. The transit data, survey data, odometer data and traffic count data are all somewhat different. But from the above, it appears that 2 billion passenger-miles traveled for transit and 44 billion passenger-vehicle miles are good as rough numbers for perspective. The number of passenger-miles traveled in passenger vehicles will be something well over 44 billion. So, statewide, transit accounts for under 5% of passenger-miles traveled. That’s an important number to grasp from a carbon emissions perspective.

From a congestion perspective the answer is different — in the most congested areas of the state, transit share is higher and its role in relieving congestion can be significant.

Published by Will Brownsberger

Will Brownsberger is State Senator from the Second Suffolk and Middlesex District.

40 replies on “Transit energy efficiency”

  1. I just heard a proposal to offer fare discounts to lower income riders. This makes a lot of sense. Public transit runs whether at capacity or not. Discounted fares would encourage higher capacity, and reduce emission per rider mile, thus satisfying the goal of lower greenhouse gas emissions.

      1. Where the T had to raise its fares recently where will they find the money if fares are reduced for soe of its riders?

          1. When you say nes sources of revenue just what are you thinking about? Gasoline tax, increased fares during rush hour, a renters tax. a real estate tax, more money from the general fund, ????

  2. I think your numbers are a bit high for passenger load factor. For buses, in 2017, I think the real number is under 12.5 given how revenue miles exclude deadhead, operator training, and maintenance test miles. The private sector is emulating public transit now, given how Uber loses $1.20 on every trip and bikeshares also lose money! With door to door bargains like that, public transit ridership is down a few percent in many transit systems, including Boston’s. Of MBTA modes, commuter rail most closely approaches the cost of driving (based on IRS mileage allowance), so could spare some charity to Worcester riders.

    1. Thanks, Mark. The numbers are straight from the federal transit database, but you are right that they exclude deadhead trips to the start of a route, so they are probably slightly generous.

      Absolutely agree we need to cut commuter rail fares. They are prohibitive for many people. I would like to see us cut T fares more broadly.

  3. Here in Cambridge we are struggling to deal with our density, but it does allow many of us to walk or bike. (I own a car, but got through nearly the whole summer on one tank of gas.)
    Dealing creatively with densification must be a companion piece to transport improvements. So too, I think, is the effort to encourage working from home. There are employers who should dust off old attitudes and help this movement expand. There is a lot of cultural change being asked of us on all fronts here. Terrible pun and all, it’s a bumpy ride right now, but we can think and educate our way out of this.
    If there are history buffs among you, here’s a great, relevantpiece:

  4. Great write up Will, plus the city lab piece was historically informative. Obviously the CO2 footprint is important and can hopefully be improved. I’ve always thought our buses are too large for the number of people riding.. The other issue is commute time. I, like Jeanne, cycle to work and can do so in ~20 minutes. If I take my car to go the same 5.5 miles from Belmont, esp now with end of summer and school starting, it can be 30-45 in and 40-50 returning. I know I’m moving to a new issue, but the Belmont cut thru to avoid Rt 2 is getting really significant. Huron can be fully backed up btwn Grove and FP Parkway. Is this something under study? Thanks

  5. This is really good information. I would like to suggest that, IMHO, public transit should be FREE – encouraging more people to put up with the longer times that it usually takes, helping people with low incomes and saving the MBTA the whole cost of collecting, counting and verifying. The tickets don’t cover the cost anyway! Sure it would require a larger amount from the city, but it should be viewed by tax payers as an expense that reduces congestion!

  6. I think the fare discounts are necessary. Could there ever be a small state gasoline tax increase (.01 to .02 cents per gallon) to subsidize a systemwide ‘no charge MBTA’ on Friday’s + Monday’s (or heaviest car usage day’s of the week). Simply to get single car drivers out of their vehicles, [change wasteful habits] and onto the MBTA. Thanks for making all of the info. available Will.

    1. I’d like to see more progressive policies and agree with Dave, fare discounts c/should be paid for with progressive taxes on carbon use (i.e. gasoline).

  7. Transit X pods have an efficiency of about 50 BTUs per passenger mile which is approximately 100 times higher efficiency than personal cars and 12 times higher efficiency than heavy rail. The pods use 100% renewable energy and significantly reduce carbon emissions. Proposals for over 100 municipalities in Massachusetts can be found at

    1. I was unaware of TX but have digested the concept and looked at some of the proposals to municipalities. They are well written, and the concept seems technically feasible, but how have your pitches been received?

      Yesterday I spoke with a Belmont Town Meeting member who recalled receiving your proposal almost 3 years ago. He told me the Selectmen rejected it for the simple reason that to clear RoWs, many trees would have to be cut down. That would create a total uproar and change the character of our streets.

      In dense urban areas where trees are far between, TX makes a lot more sense, but in the ‘burbs not so, and am wondering what you think should be done? You could set up median barriers and erect poles on them, I suppose, but entrance and egress would be tricky, if not hazardous. Eventually, I suppose if TX allows streets to be ripped up there will be plenty of room for trees, but I don’t see it happening outside of the urban core.

  8. I would like to see Public Transit adjust fares based upon expected demand. Evenings, weekends, perhaps midday perhaps could be free. This will encourage both new users to start using the system and some existing users to time shift their usage. This will spread the load on the system and improve system efficiency. Unless you can add substantial capacity making the system free during rush hour is only going to strain an already overloaded system. The rideshare companies already adjust rates based upon load to make their system more efficient. I think the MBTA should do something similar. The easiest way to add system capacity may be to encourage users to time shift.

  9. I work directly across from South Station and a couple of times a day a train will be idling on Track 1 (closest to the street) for 20+ minutes. Not only is it super loud, it’s also consuming fuel and polluting the entire time. I think trains are allowed to idle up to 30 minutes so they’re not breaking the law, but it would increase efficiency if these trains could be shut off or plugged in when they’re in the station for extended periods.

  10. These numbers include only energy & carbon cost for operating vehicles, not the energy required to build, maintain and dispose of them. I think we need a more synoptic picture.

    Nor does the data compare private electric vehicles to gas and diesel powered ones. Our EV is certainly perhaps 40% cheaper to fuel at residential rates; charging networks can cost more or less. What that amounts to in terms of carbon I don’t know. Perhaps there isn’t enough data yet, and what data I’ve seen that figures in the entire lifecycle of EVs versus conventional vehicles is scant and confusing.

    Seems to me that reducing travel by foregoing, aggregating, and sharing car trips could have considerable impact on emissions, but how to get drivers to ask themselves “Is this trip necessary, at least at this time? Could I walk, bike, or take the T instead?” But nobody is asking the public to consider such questions. They should be posted on billboards and discussed in the media.

    It turns out that a lot more of cars on the road at the turn of the previous century were EVs than now are (there were even more steam-powered ones). There’s an article/memoir/critique on my blog that discusses the history and pluses and minuses of EVs and my experience as an owner of one. On the whole it has been very positive, except for the constant surveillance, but EVs are not unique in that unfortunate respect.

    1. Thanks, Geoff.

      This is a valid line of argument. Electrifying more private vehicles and getting people to share them more (Uber-like technology could identify route synergies) is definitely a way to reduce carbon costs.

      In fact, there is data which I am looking at now which suggests that light vehicle electrification is actually a lot cheaper from a capital standpoint than electrifying transit.

  11. One of the most disturbing things I’ve learned is how all the new high-rise building construction in Boston is being done without consideration for energy efficiency. Lots of windows, which are not green. Need to establish building energy efficiency codes or we will be saddled with these inefficient buildings for decades.

  12. A radical if not mad idea to use existing technology: very large tax credits to all who commute by exclusively human-powered bicycle, or by cycle rickshaw for those not able to propel themselves; high tax-free salaries for rickshaw drivers, who would be doing socially valuable work.

  13. Thanks Will for this informative summary. Two thoughts: how about smaller buses for non-peak hours? And, how about turning off the overhead platform lights during the day at outdoor stations such as Charles St. on the Red Line? The lights are certainly unnecessary during the day.. Thanks!

    1. Thanks, MA.

      I agree about the lighting. That should be addressed.

      The pushback on smaller buses during the day is (a) most of the operating cost is the driver, so it doesn’t save much money; (b) it is costly to maintain two different fleets; (c) it is hard to store two different fleets (we have enough trouble storing one.

  14. Thanks for the numbers. Interesting that about buses. I’d assumed better.

    Aside from needing to factor in manufacturing emissions (and the grid power sources where the manufacturing happens matters), the point Geoff raised, there are a couple other points against too strictly limiting bus routes based ridership:

    1. the ability to go car-less depends on routes going to all the places you need regularly to go. I think of an earlier article where you mention affordability of the commuter rail for workers coming from gateway cities. Presumably it is unaffordable because the cost was in addition to owning a car. More comprehensive transit in those communities and better links between communites (ever look at how to go from Worcester to Fitchburg without a car? Fitchburg to Brattleboro?) might allow people to forego the car.

    2. if not buses then what, given the cost of rail and how long it takes to roll out new lines? Electric cars? Naturally, an electric bus is an improvement over an electric car for some level of ridership, similar to how a diesel bus improves on an IC car. Electric cars cause more CO2 (about 15% more than IC cars IIRC) in their manufacture so amortizing that across multiple riders also helps.

    Speed of adoption and affordability of electric cars are concerns of mine. Could, with appropriate public support, a transit agency convert and expand their bus fleet sooner than private vehicle owners convert? But then I guess there’s the ongoing problem of moving people to public transit, which seems only possible here for Red Sox games. So both tracks are needed, encouraging private electric car use and converting transit to electric while increasing ridership. Yet at the national level the Democratic Candidates don’t mention public transit at all (okay, I saw mention on Bernie Sanders’s website). Did the Democratic Convention forbid them to talk about that too;)?

    1. Really good thoughts, Mike.

      Agreed that we need to maintain a transit network to support mobility even if it is not more efficient than cars. Some cannot afford or cannot drive cars.

      Figuring out how to roll-out more electric cars is a challenge . . . not sure it will move slower or faster than transit electrification.

  15. Will,
    I can see your logic when you say that if you use the T it is carbon free because it is going to run anyway. However, there is still a carbon footprint for the T and I am wondering what the T’s baseline footprint is and if it can be broken down by bus, subway, train.
    Also, With all of the emphasis on mass transit and alternative energy why is the T (subway and trackless trolley) not running on exclusively in alternative energy? I just cannot understand what the excuses would be and who would agree with their excuses?

    1. Yes. Download the spreadsheet from the link above the chart — it has all the numbers on the T footprint.

      The T uses power from the same electrical grid we do (except in emergencies). The challenge is to make all that power green.

      1. Great, I will look at the charts again. But, what is the answer as to why the T is not using 100% alternative energy NOW? The Town of Watertown just signed an contract with one of these companies for everyone in the town so there must be plenty of it available, if the T or the state does not want to push this then why expect anyone else to?

  16. Another point: by using energy per passenger mile you are factoring out length of trips, which also relate to total emissions. Since car use implies a maximum density lower than what bus and subway can support, if cities were planned optimally, we should expect car trips, particularly in an area (un)planned around that mode, to average longer distances than buses or subway trips, particularly in a city planned around those modes. My intuition is that in Boston this is visibly and intuitively true. Talk to the people around you about where they’re driving in from or note where bus routes end compared to population density.

    2nd additional point: there’s also the lifestyle that cars and the greater sprawl supports. I don’t see so many people lugging home their 2nd big screen tv on the 86 for instance, nor am I surrounded by large houses that don’t share walls with neighbouring units in here where the car is, to paraphrase Jerry Seinfeld, an infection the city’s immune system needs to repel.

    So even if the bus might not look much better than the car when looked at narrowly, the kind of city and lifestyle the bus supports may improve its numbers when looked at more broadly.

  17. Just throwing this out – even though it’s not quite aligned with our bus, train conversation. But it’s a new discovery for me. Blue Bikes! The system has vastly improved, compared to the old Hubways. The gear mechanism is continuously variable (which, in itself, is a small miracle.) The user interface is clean and snappy. Best of all, there are stations everywhere you could possibly want to go in central Boston and Cambridge.
    So. Haven’t needed a car for one single day for the past 2 months.
    OK, Thank you.

  18. Hi,
    I’ve always had the idea that in off hours the MBTA could use smaller capacity buses. Schools use them for more sparsely used routes. Is that reasonable, or about the same amount of carbon emission?
    Thanks for everything you do.
    Liz King

  19. Thanks, Liz.
    Smaller buses at off hours probably would save carbon emissions, but here are the push backs: (a) most of the operating cost is the driver, so it doesn’t save much money; (b) it is costly to maintain two different fleets; (c) it is hard to store two different fleets (we have enough trouble storing one).

  20. Thank you for this information, Will. The T has been running diesel trains under electric catenary on the Providence line for 20 years now. I know the supposed reasons for this this, which generally reflect a lack of competence at the T. I’m glad to see the state is finally looking at leasing some electric locomotives to test out this “radical” idea. The next steps should include electrifying the Stoughton branch and the Fairmount branch. Others should follow.
    I think one reason the people in Belmont and Watertown love their trolley buses is that, as with rail lines, the wires represent a long-term commitment to the service. I know the service wouldn’t go away if the lines were converted to electric vehicles, but it’s a psychological factor.

  21. I live in Watertown and do not love the trolley buses all that much. I’ve been on several that have “derailed” and had to wait while the driver got out of the bus and reattached the bus to the wire. Also, they cannot pass each other if the buses get bunched up or if a 71 is right behind a 73 or vice-versa. And yet they emit only slightly less carbon than a regular bus (at least until generation becomes greener).

    Also, even having a rail line does not guarantee that the service won’t go away. Watertown used to host a branch of the Green Line but that got replaced by the 57 bus in the late ’60s:

    1. I’m in favor a switch to battery buses for the 71 and 73. The T has actually said they want to do that. As Greg says, the trolleys have some real limitations.

Comments are closed.