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Verbas I.,Transportation Center | Mahmassani H.,Northwestern University
Transportation Research Record | Year: 2013

This study proposes a formulation for the transit network frequency setting problem. The formulation provides an optimal allocation of resources over space and time while recognizing the existence of multiple service patterns along each bus route. Transit agencies must allocate their limited resources optimally to maximize user benefits, operator benefits, or a combination of the two. The coupling of the routes with the service patterns provided along all or portions of the routes is effectively captured, and the user perspective and the operator perspective are merged into one formulation. The service patterns may be scheduled with different subsets of stops for a given route. Users see the resulting combined route headways at the stops. The number of riders varies with the prevailing number of bus trips at a given stop, which is the combination of different pattern dispatch frequencies. Two main formulations are introduced. The first extends work of Furth and Wilson and seeks to maximize the number of riders and the total wait time savings under budget, fleet, policy headway, and bus loading constraints. The second minimizes the net cost under fleet, policy headway, bus loading, minimum ridership, and minimum wait time savings constraints. In both formulations, pattern headways in different time-of-day intervals are the decision variables. This paper provides the mathematical formulation underlying the proposed methodology, describes the solution method and implementation, and demonstrates, by example, important properties of the frequency setting problem in this context, including some that may at first appear counterintuitive. Source


Mahmassani H.,Transportation Center | Hou T.,Northwestern University | Saberi M.,Northwestern University
Transportation Research Record | Year: 2013

The existence of the network fundamental diagram (NFD) has been established at the urban network scale. It relates three traffic descriptors: speed, density, and flow. However, its deterministic nature does not convey the underlying variability within the network. In contrast, travel time reliability as a network performance descriptor is of growing concern to both the traveling public and traffic managers and policy makers. The objectives of this paper were to extend travel time reliability modeling from the link-path level to the network level and to connect overall network variability to NFD. Robust relationships between travel time variability and network density and flow rate were analytically derived, investigated, and validated with both simulated and real-world trajectory data. The distance-weighted standard deviation of travel time rate, as a measure of travel time variability, was found to increase monotonically with network density. A maximum network flow rate existed beyond which network travel time reliability deteriorated at a much faster pace. The results also suggest that these relationships are inherent network properties (signature) that are independent of demand level. The effects of en route information on the proposed relationships were also studied. The results showed that en route information reduced network travel time variability. The findings provide a strong connection between NFD and travel time variability, and this connection can be used further for modeling of network travel time reliability and assessment of measures intended to improve reliability of travel in a network. Source


Saberi M.,Building 60 | Mahmassani H.S.,Transportation Center | Hou T.,Northwestern University | Zockaie A.,Northwestern University
Transportation Research Record | Year: 2014

This paper evaluates measurement methods for traffic flow variables taken at the network level. Generalized Edie's definitions of fundamental traffic flow variables along highways are extended for considering vehicles traveling in networks. These definitions are used to characterize traffic flow in networks and form the basis for estimating relationships among network density, flow, and speed in the form of a network fundamental diagram. The method relies on three-dimensional vehicle trajectories to provide estimates of network flow, density, and speed. Such trajectories may be routinely obtained from particle-based microscopic and mesoscopic simulation models and are increasingly available from tracking devices on vehicles. Numerical results from the simulation of two networks, in Chicago, Illinois, and Salt Lake City, Utah, are presented to illustrate and validate the estimation methodology. As part of the verification process, the study confirms that the traffic flow fundamental identity (Q 5 K · V) holds at the network level only when networkwide traffic flow variables are defined consistently with Edie's definitions. Source


News Article
Site: http://news.mit.edu/topic/mitenergy-rss.xml

Could existing electric vehicles (EVs), despite their limited driving range, bring about a meaningful reduction in the greenhouse-gas emissions that are causing global climate change? Researchers at MIT have just completed the most comprehensive study yet to address this hotly debated question, and have reached a clear conclusion: Yes, they can. The study, which found that a wholesale replacement of conventional vehicles with electric ones is possible today and could play a significant role in meeting climate change mitigation goals, was published today in the journal Nature Energy by Jessika Trancik, the Atlantic Richfield Career Development Associate Professor in Energy Studies at MIT’s Institute for Data, Systems, and Society (IDSS), along with graduate student Zachary Needell, postdoc James McNerney, and recent graduate Michael Chang SM ’15. “Roughly 90 percent of the personal vehicles on the road daily could be replaced by a low-cost electric vehicle available on the market today, even if the cars can only charge overnight,” Trancik says, “which would more than meet near-term U.S. climate targets for personal vehicle travel.” Overall, when accounting for the emissions today from the power plants that provide the electricity, this would lead to an approximately 30 percent reduction in emissions from transportation. Deeper emissions cuts would be realized if power plants decarbonize over time. The team spent four years on the project, which included developing a way of integrating two huge datasets: one highly detailed set of second-by-second driving behavior based on GPS data, and another broader, more comprehensive set of national data based on travel surveys. Together, the two datasets encompass millions of trips made by drivers all around the country. The detailed GPS data was collected by state agencies in Texas, Georgia, and California, using special data loggers installed in cars to assess statewide driving patterns. The more comprehensive, but less detailed, nationwide data came from a national household transportation survey, which studied households across the country to learn about how and where people actually do their driving. The researchers needed to understand “the distances and timing of trips, the different driving behaviors, and the ambient weather conditions,” Needell says. By working out formulas to integrate the different sets of information and thereby track one-second-resolution drive cycles, the MIT researchers were able to demonstrate that the daily energy requirements of some 90 percent of personal cars on the road in the U.S. could be met by today’s EVs, with their current ranges, at an overall cost to their owners — including both purchase and operating costs — that would be no greater than that of conventional internal-combustion vehicles. The team looked at once-daily charging, at home or at work, in order to study the adoption potential given today’s charging infrastructure. What’s more, such a large-scale replacement would be sufficient to meet the nation’s stated near-term emissions-reduction targets for personal vehicles’ share of the transportation sector — a sector that accounts for about a third of the nation’s overall greenhouse gas emissions, with a majority of emissions from privately owned, light-duty vehicles. While EVs have many devotees, they also have a large number of critics, who cite range anxiety as a barrier to transportation electrification. “This is an issue where common sense can lead to strongly opposing views,” Trancik says. “Many seem to feel strongly that the potential is small, and the rest are convinced that is it large.” “Developing the concepts and mathematical models required for a testable, quantitative analysis is helpful in these situations, where so much is at stake,” she adds. Those who feel the potential is small cite the premium prices of many EVs available today, such as the highly rated but expensive Tesla models, and the still-limited distance that lower-cost EVs can drive on a single charge, compared to the range of a gasoline car on one tank of gas. The lack of available charging infrastructure in many places, and the much greater amount of time required to recharge a car compared to simply filling a gas tank have also been cited as drawbacks. But the team found that the vast majority of cars on the road consume no more energy in a day than the battery energy capacity in affordable EVs available today. These numbers represent a scenario in which people would do most of their recharging overnight at home, or during the day at work, so for such trips the lack of infrastructure was not really a concern. Vehicles such as the Ford Focus Electric or the Nissan Leaf — whose sticker prices are still higher than those of conventional cars, but whose overall lifetime costs end up being comparable because of lower maintenance and operating costs — would be adequate to meet the needs of the vast majority of U.S. drivers. The study cautions that for EV ownership to rise to high levels, the needs of drivers have to be met on all days. For days on which energy consumption is higher, such as for vacations, or days when an intensive need for heating or cooling would sharply curb the EV’s distance range, driving needs could be met by using a different car (in a two-car home), or by renting, or using a car-sharing service. The study highlights the important role that car sharing of internal combustion engine vehicles could play in driving electrification. Car sharing should be very convenient for this to work, Trancik says, and requires further business model innovation. Additionally, the days on which alternatives are needed should be known to drivers in advance —information that the team’s model “TripEnergy” is able to provide. Even as batteries improve, there will continue to be a small number of high-energy days that exceed the range provided by electric vehicles. For these days, other powertrain technologies will likely be needed. The study helps policy-makers to quantify the “returns” to improving batteries through investing in research, for example, and the gap that will need to be filled by other kinds of cars, such as those fueled by low-emissions biofuels or hydrogen, to reach very low emissions levels for the transportation sector. Another important finding from the study was that the potential for shifting to EVs is fairly uniform for different parts of the country. “The adoption potential of electric vehicles is remarkably similar across cities,” Trancik says, “from dense urban areas like New York, to sprawling cities like Houston. This goes against the view that electric vehicles — at least affordable ones, which have limited range — only really work in dense urban centers.” Jeremy J. Michalek, a professor of engineering and public policy at Carnegie Mellon University who was not involved in this study, says the MIT team’s integration of the GPS and national survey data is a new approach “highlighting the novel idea that regional differences in range requirements are minor for most vehicle-day trips but increase as we move into higher-range trips.” The study, he says, is both “interesting and useful.” The work was supported by the New England University Transportation Center at MIT, the MIT Leading Technology and Policy Initiative, the Singapore-MIT Alliance for Research and Technology, the Charles E. Reed Faculty Initiatives Fund, and the MIT Energy Initiative.


News Article | August 16, 2016
Site: http://www.theenergycollective.com/rss/all

Could existing electric vehicles (EVs), despite their limited driving range, bring about a meaningful reduction in the greenhouse-gas emissions that are causing global climate change? Researchers at MIT have just completed the most comprehensive study yet to address this hotly debated question, and have reached a clear conclusion: Yes, they can. The study, which found that a wholesale replacement of conventional vehicles with electric ones is possible today and could play a significant role in meeting climate change mitigation goals, was published today in the journal Nature Energy by Jessika Trancik, the Atlantic Richfield Career Development Associate Professor in Energy Studies at MIT’s Institute for Data, Systems, and Society (IDSS), along with graduate student Zachary Needell, postdoc James McNerney, and recent graduate Michael Chang SM ’15. “Roughly 90 percent of the personal vehicles on the road daily could be replaced by a low-cost electric vehicle available on the market today, even if the cars can only charge overnight,” Trancik says, “which would more than meet near-term U.S. climate targets for personal vehicle travel.” Overall, when accounting for the emissions today from the power plants that provide the electricity, this would lead to an approximately 30 percent reduction in emissions from transportation. Deeper emissions cuts would be realized if power plants decarbonize over time. The team spent four years on the project, which included developing a way of integrating two huge datasets: one highly detailed set of second-by-second driving behavior based on GPS data, and another broader, more comprehensive set of national data based on travel surveys. Together, the two datasets encompass millions of trips made by drivers all around the country. The detailed GPS data was collected by state agencies in Texas, Georgia, and California, using special data loggers installed in cars to assess statewide driving patterns. The more comprehensive, but less detailed, nationwide data came from a national household transportation survey, which studied households across the country to learn about how and where people actually do their driving. The researchers needed to understand “the distances and timing of trips, the different driving behaviors, and the ambient weather conditions,” Needell says. By working out formulas to integrate the different sets of information and thereby track one-second-resolution drive cycles, the MIT researchers were able to demonstrate that the daily energy requirements of some 90 percent of personal cars on the road in the U.S. could be met by today’s EVs, with their current ranges, at an overall cost to their owners — including both purchase and operating costs — that would be no greater than that of conventional internal-combustion vehicles. The team looked at once-daily charging, at home or at work, in order to study the adoption potential given today’s charging infrastructure. What’s more, such a large-scale replacement would be sufficient to meet the nation’s stated near-term emissions-reduction targets for personal vehicles’ share of the transportation sector — a sector that accounts for about a third of the nation’s overall greenhouse gas emissions, with a majority of emissions from privately owned, light-duty vehicles. While EVs have many devotees, they also have a large number of critics, who cite range anxiety as a barrier to transportation electrification. “This is an issue where common sense can lead to strongly opposing views,” Trancik says. “Many seem to feel strongly that the potential is small, and the rest are convinced that is it large.” “Developing the concepts and mathematical models required for a testable, quantitative analysis is helpful in these situations, where so much is at stake,” she adds. Those who feel the potential is small cite the premium prices of many EVs available today, such as the highly rated but expensive Tesla models, and the still-limited distance that lower-cost EVs can drive on a single charge, compared to the range of a gasoline car on one tank of gas. The lack of available charging infrastructure in many places, and the much greater amount of time required to recharge a car compared to simply filling a gas tank have also been cited as drawbacks. But the team found that the vast majority of cars on the road consume no more energy in a day than the battery energy capacity in affordable EVs available today. These numbers represent a scenario in which people would do most of their recharging overnight at home, or during the day at work, so for such trips the lack of infrastructure was not really a concern. Vehicles such as the Ford Focus Electric or the Nissan Leaf — whose sticker prices are still higher than those of conventional cars, but whose overall lifetime costs end up being comparable because of lower maintenance and operating costs — would be adequate to meet the needs of the vast majority of U.S. drivers. The study cautions that for EV ownership to rise to high levels, the needs of drivers have to be met on all days. For days on which energy consumption is higher, such as for vacations, or days when an intensive need for heating or cooling would sharply curb the EV’s distance range, driving needs could be met by using a different car (in a two-car home), or by renting, or using a car-sharing service. The study highlights the important role that car sharing of internal combustion engine vehicles could play in driving electrification. Car sharing should be very convenient for this to work, Trancik says, and requires further business model innovation. Additionally, the days on which alternatives are needed should be known to drivers in advance —information that the team’s model “TripEnergy” is able to provide. Even as batteries improve, there will continue to be a small number of high-energy days that exceed the range provided by electric vehicles. For these days, other powertrain technologies will likely be needed. The study helps policy-makers to quantify the “returns” to improving batteries through investing in research, for example, and the gap that will need to be filled by other kinds of cars, such as those fueled by low-emissions biofuels or hydrogen, to reach very low emissions levels for the transportation sector. Another important finding from the study was that the potential for shifting to EVs is fairly uniform for different parts of the country. “The adoption potential of electric vehicles is remarkably similar across cities,” Trancik says, “from dense urban areas like New York, to sprawling cities like Houston. This goes against the view that electric vehicles — at least affordable ones, which have limited range — only really work in dense urban centers.” Jeremy J. Michalek, a professor of engineering and public policy at Carnegie Mellon University who was not involved in this study, says the MIT team’s integration of the GPS and national survey data is a new approach “highlighting the novel idea that regional differences in range requirements are minor for most vehicle-day trips but increase as we move into higher-range trips.” The study, he says, is both “interesting and useful.” The work was supported by the New England University Transportation Center at MIT, the MIT Leading Technology and Policy Initiative, the Singapore-MIT Alliance for Research and Technology, the Charles E. Reed Faculty Initiatives Fund, and the MIT Energy Initiative.

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