This study investigates the isolated and combined effects of roadway grade, passenger load, network congestion level, and
fuel type on transit bus emissions. First, the isolated and combined effects of the different variables were analyzed through a structured sensitivity analysis. Next, the effects of disregarding grade and PLF were assessed under realistically occurring
combinations of these variables. In both exercises, the Cote des Neiges corridor in Montreal, Canada was chosen because
it features a wide variation in traffic, grades and passenger activity across its different links. In both the sensitivity analysis
and case-study, every model result represents an average of six different model runs (under six different random seeds).
Our investigation allowed us to derive a number of conclusions which are useful under two main applications: (1) when
corridor-level emission estimations are conducted by transit providers in order to evaluate total and per passenger emissions, and (2) in the selection of corridors when a number of new and alternative-fuelled buses are to be deployed. Our most
intuitive finding relates to the effect of speed; indeed, buses generate higher emissions in congested networks due to lower
speeds, higher frequency of acceleration and deceleration events, and higher dwell times at stops. The effects of grade are
different at different network speeds and positive grades have a higher effect on emissions than negative grades. Moreover,
the effect of negative grade is cancelled out by the randomness in traffic patterns. The consideration of passenger load also
affects bus emissions but the influence of load is not as strong as it is for grade. Load effects are more important under high
positive grades and if dwell emissions are considered along with running emissions, the effect becomes larger. The consideration of passenger load is important when emissions are estimated on a per passenger basis. When the number of passengers exceeds the seating capacity of the bus, additional passengers no longer contribute noticeably to reduce per passenger
emissions. In comparing CNG and ULSD, we observe that the benefit of using CNG increases with an increase in network level
congestion. Also the CNG benefit decreases with an increase in grade and passenger load.
This study investigates the isolated and combined effects of roadway grade, passenger load, network congestion level, andfuel type on transit bus emissions. First, the isolated and combined effects of the different variables were analyzed through a structured sensitivity analysis. Next, the effects of disregarding grade and PLF were assessed under realistically occurringcombinations of these variables. In both exercises, the Cote des Neiges corridor in Montreal, Canada was chosen becauseit features a wide variation in traffic, grades and passenger activity across its different links. In both the sensitivity analysisand case-study, every model result represents an average of six different model runs (under six different random seeds).Our investigation allowed us to derive a number of conclusions which are useful under two main applications: (1) whencorridor-level emission estimations are conducted by transit providers in order to evaluate total and per passenger emissions, and (2) in the selection of corridors when a number of new and alternative-fuelled buses are to be deployed. Our mostintuitive finding relates to the effect of speed; indeed, buses generate higher emissions in congested networks due to lowerspeeds, higher frequency of acceleration and deceleration events, and higher dwell times at stops. The effects of grade aredifferent at different network speeds and positive grades have a higher effect on emissions than negative grades. Moreover,the effect of negative grade is cancelled out by the randomness in traffic patterns. The consideration of passenger load also
affects bus emissions but the influence of load is not as strong as it is for grade. Load effects are more important under high
positive grades and if dwell emissions are considered along with running emissions, the effect becomes larger. The consideration of passenger load is important when emissions are estimated on a per passenger basis. When the number of passengers exceeds the seating capacity of the bus, additional passengers no longer contribute noticeably to reduce per passenger
emissions. In comparing CNG and ULSD, we observe that the benefit of using CNG increases with an increase in network level
congestion. Also the CNG benefit decreases with an increase in grade and passenger load.
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