posted on 2014-11-18, 00:00authored byMichael N. Taptich, Arpad Horvath
The
life-cycle output (e.g., level of service) of infrastructure
systems heavily influences their normalized environmental footprint.
Many studies and tools calculate emission factors based on average
productivity; however, the performance of these systems varies over
time and space. We evaluate the appropriate use of emission factors
based on average levels of service by comparing them to those reflecting
a distribution of system outputs. For the provision of truck and bus
services where fuel economy is assumed constant over levels of service,
emission factor estimation biases, described by Jensen’s inequality,
always result in larger-than-expected environmental impacts (3%–400%)
and depend strongly on the variability and skew of truck payloads
and bus ridership. Well-to-wheel greenhouse gas emission factors for
diesel trucks in California range from 87 to 1,500 g of CO2 equivalents per ton-km, depending on the size and type of trucks
and the services performed. Along a bus route in San Francisco, well-to-wheel
emission factors ranged between 53 and 940 g of CO2 equivalents
per passenger-km. The use of biased emission factors can have profound
effects on various policy decisions. If average emission rates must
be used, reflecting a distribution of productivity can reduce emission
factor biases.