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 Subsystem||BTU/ |
passenger mile, 2016
|Pounds CO2/ 1000 passenger miles, 2016||NOTE: 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)||1,248||325||BTU/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)||1,169||305||BTU/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)||2,525||409||BTU/ppm checks to TEDB Figure 2.09, Energy Intensity of Commuter Rail Systems, 2016. Carbon emissions based on 2017 in-state generation efficiency, adjusted for line losses.|
|Motor Buses||4,057||580||Average 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 Buses||2,008||523||No TEDB comparison. Carbon emissions based on 2017 in-state generation efficiency, adjusted for line losses.|
|Single Occupancy Vehicle at 25 mpg||5000||784||TEDB 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 Ride||15,010||2,353||No 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.|
|Ferry||11,760||1,903||No 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.