Friday, August 31, 2007

CO2 for transportation: crunching my personal numbers

I had an email exchange with my friend V. about my blog entry on environmental awareness in Australia, and it inspired me to do a mini-research project: I calculated, from scratch, the carbon dioxide emissions for different modes of transportation that I use. There are lots of carbon footprint calculators on the web that purport to do this, but they are not always transparent about their methodology. Also, by doing the calculation myself, I can tailor it to real-life transportation scenarios that I have used in everyday life. The results are given in pounds of CO2 per mile per passenger:

Transportation ModeVehicleCO2 emissions (lb/mi/passenger)Scenario
Car1999 Honda Civic DX0.145-0.725Personal driving, 1-5 passengers.
BusNew Flyer D60LF Bus0.29-0.58VTA Route 22 on El Camino, Stanford to Sunnyvale. Extended-length bus at 25%-50% capacity.
AirBoeing 747-4000.365United Airlines flight 870, Sydney to San Francisco. Long-haul flight at full capacity.
What I learned from doing this:

We always hear about how bad air travel is for the environment. Surprisingly, it turns out that a car carrying two people emits about the same amount of CO2 per passenger-mile as a full 747 on a long-haul flight...however, the real impact of air travel is due to the distances traveled. The distances involved in flying are so much greater than those for driving - by flying roundtrip to Australia, I traveled more than double the miles I'll probably drive this entire year. According to the climate calculator at, short-haul flights emit more CO2 per passenger-mile than long-haul flights, but of course they also cover less total distance.

Passenger occupancy matters. As seen from the table, solo driving is not very good per passenger-mile, but once you start to pack people into your car, things start looking better. The same goes for the bus - if it's pretty empty, it's not much better than taking your car. This has implications for transit policy.

To sum up my learnings, measuring emissions in pounds CO2 per mile per passenger is a useful yardstick for apples-to-apples comparisons, but to understand total CO2 emissions, one must also compare and take into account the distances involved. Also, passenger occupancy can make all the difference in whether mass transit is better than driving.

I wanted to crunch numbers for some other modes of transportation, but either couldn't find data or ran out of steam. My spouse was curious about the numbers for Caltrain, motorcycles, and motorized bikes, but I think I'll leave that as an exercise for him :) Also, I wanted to do regular-size buses and Amtrak trains, but was unable to find good source data after Googling for a few hours. For a ballpark figure for buses, though, the Milwaukee County Transit System states that the average diesel-fueled bus in their fleet gets 4.5 mpg. Diesel has an emissions factor of 22.384 lb/gal, so assuming there are 15 people on the bus, you'd get 22.384 lb/gal ÷ 4.5 mpg ÷ 15 passengers = 0.33 pounds per passenger per mile.

Wikipedia has a table comparing fuel efficiency for different transportation modes. However, with all the different vehicles using different fuels and different assumptions (like driving traffic patterns and passenger loads) and then translating all of that into CO2 emissions, I wouldn't buy into that table unless I'd scrutinized how they came up with their numbers.

The gory details of the car, bus, and plane calculations follow.

Personal driving:
1999 Honda Civic DX Automatic Transmission

Combined fuel efficiency:27 mpg a
CO2 emissions factor:19.564 lb/gal b

CO2 per mile per passenger (1 passenger) = 19.564 lb/gal ÷ 27 mpg ÷ 1 passenger = 0.725 pounds per mile per passenger

CO2 per mile per passenger (5 passengers) = 19.564 lb/gal ÷ 27 mpg ÷ 5 passengers = 0.145 pounds per mile per passenger

VTA Route 22 on El Camino, Stanford to Sunnyvale:
New Flyer D60LF Bus c

Seats:62 d
Passenger load factor:0.25-0.50 g
Fuel efficiency:2.50 mpg e
Fuel type:Diesel f
CO2 emissions factor:22.384 lb/gal b

CO2 per mile per passenger (0.25 load factor) = 22.384 lb/gal ÷ 2.50 mpg ÷ (62 seats × 0.25 load factor) = 0.58 pounds per mile per passenger

CO2 per mile per passenger (0.5 load factor) = 22.384 lb/gal ÷ 2.50 mpg ÷ (62 seats × 0.5 load factor) = 0.29 pounds per mile per passenger

United Airlines flight 870, Sydney to San Francisco:
Boeing 747-400

Passengers (3-class configuration):416 h
Maximum fuel capacity:216,840 L h
Maximum range:13,450 km h
Fuel type:Jet A-1 i
Fuel density at 15 °C (60 °F):0.775-0.840 kg/L i
CO2 emissions factor:3.16 j

Maximum total CO2 emitted = 216,840 L × 0.840 kg/L k × 2.204623 lb/kg × 3.16 = 1,268,937.118 pounds

Maximum distance traveled = 13,460 km × 0.6213712 mi/km = 8363.656 miles

CO2 per mile per passenger (at full capacity) = 1,268,937.118 lb ÷ 8363.656 mi ÷ 416 passengers = 0.365 pounds per mile per passenger

a Fuel Economy Guide, US EPA
b Fuel and Energy Source Codes and Emission Coefficients, US Department of Energy
c Leon Norrington, New Flyer Articulated Coaches, VTA Information Bus Enthusiasts Stop
d Diesel Vehicle Specifications, New Flyer
e Table 8, King County Metro Transit Hybrid Articulated Buses: Final Evaluation Results, National Renewable Energy Laboratory, US Department of Energy
f Appendix G, FY 2006-2015 Short Range Transit Plan, Santa Clara Valley Transit Authority
g Load factor range of 0.25-0.5 is a guesstimate based on my own personal observations.
h Technical Characteristics - Boeing 747-400, Boeing
i Jet fuel, Wikipedia
j Fuel Efficiency, International Air Transport Association
k Using the maximum density value for the most conservative estimate.

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