Study suggests operational optimization can maximize the health and climate benefits of public transit investments in new bus technologies
A study by a team from the University of British Columbia and Metro Vancouver suggests that optimized operational control strategies for transit bus fleets ultimately offer transit agencies a way to maximize the benefits of their capital investments in new, cleaner technologies.
In their paper published in the ACS journal Environmental Science & Technology, they note that the evolution in bus technologies, particularly with respect to controlling pollutants that impact health—such as PM—combined with capital investments by transit agencies in these technologies, have resulted in the potential for large differences in emission factors within transit bus fleets. Operational optimization strategies such as vehicle assignment and scheduling can exploit this heterogeneous mix and minimize the climate and health impacts as well as operating costs of transit systems with minimal capital expenditure, they suggest.
New technology and operational control strategies are complementary and transit agencies should pursue both types of control strategies, they conclude.
While a large number of studies have evaluated the potential of technological and capital control strategies (e.g., aftertreatment devices) to reduce these impacts, surprisingly few have investigated how to minimize these impacts by incorporating them into the operational planning of public transportation systems, and more specifically, as objectives in the vehicle scheduling problem.
In addition to this research gap, many previous studies suffer from one or more of the following limitations. First, they have typically analyzed public health and climate impacts independently and not explicitly considered how control strategies targeting one impact may affect another. Such approaches have been shown to lead to detrimental and unintended outcomes...Second, past studies have typically employed regional scale analyses that focus on total emissions rather than exposure (i.e., the contact of pollutants and people in space and time). A number of studies have shown that regional scale analyses (i.e., analyses that focus on the relationship between total emissions, ambient pollutant concentrations, and health impacts) may underestimate exposure and health effects because they do not account for variability in the intraregional spatial and temporal distribution of pollutants and people and the relationships between them.
...Finally, of the studies that have specifically explored incorporating climate and health impacts as objectives in the vehicle scheduling problem, none have previously considered the impacts of emerging pollutants of concern such as black carbon (BC) or the trade-offs between conflicting objectives in detail.—Gouge et al.
The goal of the research team was to evaluate the potential of operational (in contrast to capital) control strategies to reduce the public health and climate impacts of transit systems in an integrated framework. To do this, they developed climate and health impact indicators as well as operating cost estimates and incorporated them as objectives in a “vehicle assignment optimization problem”.
The vehicle assignment problem—i.e., how vehicles are assigned to routes—represents one dimension of the more general vehicle scheduling problem.
In their study, Gouge et al. explored how heterogeneity in the emission levels of different bus technologies and the exposure potential of bus routes, quantified at an intraregional scale, can be exploited by vehicle assignment optimization to minimize impacts. They also investigated the implications of applying social cost−benefit analysis to evaluate trade-offs between conflicting objectives.
The study used a real-world case study of the transit system in Vancouver, Canada. Distance-based emissions factors, fuel consumption, and fuel and maintenance costs were estimated for diesel and CNG buses. Climate impacts of both long-lived compounds including CO2 and methane as well as short-lived compounds including black and organic carbon (OC), both constituents of particulate matter (PM), were quantified using their estimated global warming potential (GWP).
The total intake of PM2.5 emissions, calculated using the estimated intake fraction of the bus routes, was used as the public health impact indicator. Total NOxemissions were also considered. The estimated health, climate, and operating costs and impacts were incorporated into a vehicle assignment optimization model, and scenarios were developed to explore the implication of different optimization objectives (e.g., minimizing climate impacts).
Among their findings were:
The magnitude of the benefits of operational optimization are dependent on heterogeneity in the emission factors of the bus fleet, as well as characteristics of the transit system and the region. The greater the heterogeneity, the greater the potential benefits.
Regional scale approaches are biased and underestimate exposure by 10−13% compared to intraregional scale approaches.
In general, the benefits of operational optimization would be greatest for large heterogeneous bus fleets that operate in regions where there is significant variably in the population density.
In relative terms, vehicle assignment had a much greater effect on health impacts than either climate impacts or operating costs.
Although it is difficult to identify an optimal solution with certainty, unless there are significant biases in the model or the relative value of the impacts, it is unlikely that the optimization would lead to grossly non-optimal solutions.
Brian Gouge, Hadi Dowlatabadi, and Francis J. Ries (2013) Minimizing the Health and Climate Impacts of Emissions from Heavy-Duty Public Transportation Bus Fleets through Operational Optimization. Environmental Science & Technology doi: 10.1021/es304079p