Outside of urban areas, motorcycles and scooters are worse for global warming than cars.
In urban areas, motorcycles are worse than carpooling, but better than driving a car solo.

Emissions from Driving

On current.com, someone asked if driving motorcycles is better for the environment than driving cars. I looked it up and was surprised to learn that motorcycles & scooters often have worse emissions than cars, even though they get better gas mileage. The reason is motorcycles and scooters have worse pollution control standards, so even though they emit less CO2, the emissions of more potent greenhouse gases count for more than the CO2 emissions. The nuanced answer (given in the table below) is that 2-wheelers are better than single-driver cars in urban areas and worse in rural or highway settings. If you are comparing a two-wheeler to carpooling, then the motorcycles are worse even in urban areas. Another factor is that carbon monoxide (CO), nitrous oxides (NOx), and unburnt gas (HC's) have bad human health impacts. So heavy emissions of those in a city is bad news.

Greenhouse Warming Potential in grams CO2-equivalent per passenger-kilometer
Urban Rural Highway
Car - driving solo 310 180 220
Motorcycles & Scooters 260 190 330
Car - driving with one passenger 155 90 110
Car - driving with three passengers 78 45 55

The calculations are in this spreadsheet. Note that these numbers are just the averages of 8 scooters & motorcycles. Some of the 2-wheelers they tested did a lot better than others.

I got the data from: "Ana-Marija Vasic and Martin Weilenmann. Comparison of Real-World Emissions from Two-Wheelers and Passenger Cars. Environ. Sci. Technol., 2006, 40, (1), pp 149-154. Available online at: http://pubs.acs.org/cgi-bin/article.cgi/esthag/2006/40/i01/html/es0481023.html." I also made a local copy for folks without free access to that journal.

Emissions from Manufacturing

Even looking at these emissions, you'd think that the construction of a motorcycle would have less environmental impact, - on average - than a car. I thought so anyway. I looked it up on eiolca.net and got another surprise: motorcycles are worse than cars. A $15 grand car on average generates 9.41 metric tonnes of CO2-equivalent during construction. A $15 grand motorcycle generates 13.7. It's the same kind of thing when I looked at toxic releases: motorcycles were worse than cars.

These results are crazy, huh? It must be possible for scooters & motorcycles to have less impact than cars. I suspect these differences are because car companies historically got more pressure to have lower emisisons than motorcycle companies. Consequently, the auto industry has improved emissions control technologies in cars over the years while the motorcycle industry hasn't.

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