GREENHOUSE GAS EMISSION BASELINES ■ 31Determining GHG emission from mobile sources poses diff erent chal-
lenges than with stationary consumption. For road transportation, questions
are asked as to whether travel outside of the urban region, that is, by com-
muters, should be included. Th is is a moot issue for metropolitan regions, but
signifi cant when determining emissions from central cities. In the city of Paris,
for example, internal automobile trips generate emissions of 3,670 kilotons
of carbon dioxide equivalent (CO 2 e), and trips with origins or destinations
outside of the city contribute a further 2,862 kilotons of CO 2 e (Mairie de Paris
2009); these are life-cycle emissions discussed further below. Nevertheless, for
all the urban regions considered in table 2.2, tailpipe emissions within the
urban region were what was quantifi ed, so consensus is seen here. A further
issue, however, is the means by which travel activity data are determined—
an important issue to address given that GHGs from road transportation can
account for more than 30 percent (50 percent in Sacramento) of emissions in
some North American urban regions.
Th e IPCC guidelines on mobile combustion recognize two approaches for
quantifying emissions for road transportation: (1) based on quantity of fuel
sold and (2) from vehicle kilometers traveled (VKT). Approach 1 is preferred
for CO 2 emissions, because it is far more accurate. Indeed, for reasons of data
availability, consistency, and the typically small size of cross-border traffi c, the
use of fuel sales to calculate CO 2 emissions prevails over the strict application
of the national territory (IPCC 2006, vol. 2, section 1.28). Emissions of CH 4
and N 2 O from road transportation are, however, dependent on the age and
technology of vehicles, as well as the number of cold starts; hence, approach 2
is preferred for CH 4 and N 2 O.
To quantify GHG emissions from road transportation in urban areas, both
of the approaches have been used (for the three GHGs associated with energy:
CO 2 , CH 4 , and N 2 O) and a third approach involving scaling of fuel use from
wider regions, such as states or provinces (table 2.2). Several potential pitfalls
are seen here. Fuel sales data are not always available for urban areas—and
even if one can fi nd such data, an implicit assumption is that the fuel purchased
within the region is representative of activity within the region. Th is approach
may be considered compatible with the IPCC guidelines, which suggest the
use of fuel sales (although this may be more appropriate on a national basis).
Meanwhile means of determining VKT may be inconsistent between cities
because of diff erences in computer modeling, surveying, or vehicle-counting
techniques. Nevertheless, by using multiple approaches for Bangkok, New York
City, and Greater Toronto, diff erences between the three approaches have been
shown to be less than 5 percent (Kennedy and others 2010).
Moving to emissions from air transportation, three distinct alternatives
have been used for urban areas: