Emissions-Avoided This section describes methodology for estimating the reduced emissions costs on the road segment under consideration. Emissions can be classified into three types: hydrocarbons (HC), carbon monoxide (CO), and nitrous oxide (NOx). Emission rate lookup table for each emission type is given in NCHRP-B (1999). The model uses this table to obtain emission rates at the average vehicle speed.
The analysis applies only to the period associated with peak volume on the roadway. In the current formulation, the vehicle type is restricted to automobiles, and the emissions type is restricted to hydrocarbons.
The average vehicle speed before the implementation of the project, and the improvement in speed due to the project are not known with certainty. The current formulation assumes that that the average vehicle speed during peak hours before the project implementation is 30 miles per hour. Emission rates depend on vehicle speed and are estimated using Table 6. The annual emission costs are calculated using the emission rates as follows:
is the emission rate in pounds of hydrocarbons per thousand 
is the emission cost in dollars per ton of hydrocarbon