The TRansportation and Air Quality (TRAQ) research group

A microscopic simulation of transit bus emissions under transit signal priority and alternative technology

This study aims to quantify transit bus emissions under varying traffic operations as well as explore the effect of alternative technology. It evaluates whether significant emission reductions can be achieved through operational improvements alone as well as the potential of alternative technology under varying traffic conditions. Our research is set in Montreal, Canada where bus operations along a busy transit corridor are simulated in the less congested northbound (NB) and more congested southbound (SB) direction. Instantaneous bus speed profiles are then used to simulate emissions using the USEPA's Motor Vehicle Emission Simulator (MOVES) fit with local input data describing the vehicle fleet and ambient conditions. We evaluate the effects of several transit improvement scenarios including transit signal priority, bus stop relocation, and queue jumper lane. We also simulate emissions for two different fuels; diesel (currently used) as well as compressed natural gas. Finally, the combination of different fuels and transit operating conditions are compared. Emissions are estimated for GHG (in CO2-eq) and fine particulate matter (PM2.5).

We observe that Scenario 1 which entails TSP at each intersection reduces CO2-eq. emissions by 13.49% in the SB direction and 5.91% in the NB direction. This implies that TSP is more effective as congestion levels rise. In fact, in the SB direction, TSP alone provides higher emission reductions than switching to CNG technology (13.49% for TSP vs. 11.75% for CNG). The implementation of CNG in addition to TSP further reduces emissions to 22.66% in the SB direction and 14% in the NB direction.

In Scenario 2, relocating bus stops to mid-block reduces base-case emissions by 8.92% in the SB direction, a smaller reduction than the one achieved with TSP. However, this strategy does not lead to emission reductions in the NB direction (a slight increase of 0.47%). Indeed, in congested situations, and with near-side bus stops, when the signal is in red phase, the bus has to wait longer producing more emissions. Relocating bus-stops to mid-blocks would reduce the variation of speeds hence reducing emissions. In the NB direction, however, where congestion levels are lower, relocating bus stops to mid-block can lead to an increase in bus idle times (bus idles at the stop and the intersection) and hence emissions.

In Scenario 3, the relocation of bus-stops in addition to TSP reduces emissions by 12.6% in the SB direction and 2.09% in the NB direction. Compared to scenario 2, the introduction of TSP can improve the overall performance of bus stop relocation; however, TSP alone remains a more appropriate measure with higher emissions reductions.

Queue jumper lanes in Scenario 4 achieve important reductions in the SB direction (14.73%) however they only achieve minimal reductions in the less congested NB direction (1.17%). The combination of TSP, queue jumper lanes and bus-stop relocation in Scenario 5 seems to achieve the highest emission reductions in the SB direction (17.61%) but small reductions than TSP alone in the NB direction (3.64%).