New publication: Accessibility with time and resource constraints

Mahmoudi, M., Song, Y., Miller, H.J. and Zhou, X.  (2019) “Accessibility with time and resource constraints: Computing hyper-prisms for sustainable transportation planning,” Computers, Environment and Urban Systems, 73, 171-183

Abstract

Accessibility is the ease of obtaining desired destinations, activities, or services in an environment. A common accessibility measure in basic and applied transportation science is the space-time prism (STP) and the network-time prisms (NTPs): these are the envelopes of all possible paths between two locations and times in planar space and transportation networks, respectively. STPs and NTPs focus on time as the scarce resource limiting accessibility. However, other resource constraints can constrain space-time accessibility, such as limits or “budgets” for energy, emissions, or monetary expenses. This paper extends NTPs to include other resource constraints in addition to time. Network-based resource hyper-prisms (RHPs) incorporate other resource constraints into NTP, capturing the trade-offs between time and other resources in determining space-time accessibility. We conceptualize RHPs as a constrained optimization problem and develop a forward and backward resource-dependent time-dependent dynamic programming to determine the boundaries of a RHP given time and other resource budgets. We illustrate our approach using the Chicago sketch network (with 933 nodes and 2967 links) for the use case of an individual with an internal combustion engine vehicle and a carbon emission budget and using portions of Washington, D.C. and Baltimore networks (with 12,145 nodes and 30,697 links) for the use case of siting electric vehicle charging stations to maximize regional accessibility.

Keywords

Resource hyper-prisms; Space-time prisms; Accessibility; Sustainable transportation; Dynamic programming

Green accessibility: Estimating the environmental costs of network-time prisms

Network-time prisms are powerful measures of space-time accessibility within transportation networks.  However, they fail to capture the environmental costs of potential mobility.  In this paper, we present a method for estimating the expected energy consumption and emissions associated with network time prisms.  We also verify our method using data from instrumented vehicles moving within an experimental network-time prism in the Phoenix AZ road network.

Song, Y., Miller, H.J., Stempihar, J. and Zhou, X. (2017), “Green accessibility: Estimating the environmental costs of network-time prisms for sustainable transportation planning,” Journal of Transport Geography, 64, 109-119.

Abstract.  Accessibility, or the ease to participate in activities and obtain resources in a given environment, is crucial for evaluating transportation systems. Greater accessibility is often achieved by increasing individuals’ potential mobility. However, potential mobility, if realized by motorized modes, can also generate negative environmental impacts such as fossil fuel consumption and greenhouse gas (GHG) emissions. While the negative environmental impacts of greater mobility are acknowledged, there has been a lack of research to validate those impacts using empirical data, especially considering variations in individuals’ mobility levels. This paper presents a method for estimating the expected environmental costs of accessibility represented by a network-time prism (NTP). A NTP delimits all accessible locations within a network and the available time for an individual to present at each location given a scheduled trip origin and destination, a time budget and the maximum achievable speeds along network edges. Estimating the expected environmental costs of a NTP involves three steps: (1) semi-Markov techniques to simulate the probabilities to move along network edges at given times; (2) the speed profiles for reachable edges, and (3) a cost function that translates speeds into environmental impacts. We focus on air quality and employ the motor vehicle emission simulator MOVESLite to estimate the CO2 emissions at both the edge and prism levels. We calibrate and validate the methods for experimental NTPs defined within the Phoenix, AZ, USA road and highway network using vehicles instrumented with GPS-enabled onboard diagnostic devices (OBD). We demonstrate the effectiveness of our method through two scenarios and investigate the impact of changes in mobility levels on the expected CO2 emissions associated with the experimental NTPs.

Keywords: Space-time accessibility; Network-time prism; Emissions

Road building, traffic safety, oil by rail and air quality: A brimming cornucopia of transportation news

Some days its seems like it rains transportation news.  Today is one of those days.  I’m going to be efficient (read: lazy) and provide a single round-up of several interesting news items:

  1. People are driving less so should we stop building new roads? As the Atlantic Cities reports, vehicle miles traveled in the US is leveling, perhaps declining.  When will we stop increasing road capacity and invest in alternative modes?  (Thanks to Michael Widener for this post.)
  2. This week in my Sustainable Transportation course, we talked about safety.  Lo and behold, a tale of two “cities” spews forth from cyberspace – a map of NYC traffic fatalities and injuries and an article from The Economist about why Sweden has so few roads deaths.  (The Economist article notes that NYC is trying to match Sweden’s success with its Vision Zero Initiative.  One can see why.)
  3. Meanwhile in upstate New York, there is increasing concern about the amount of oil traveling by rail instead of pipelines due to the domestic energy boom.  [NYT: Bakken Crude, Rolling Through Albany].   ‘ “Albany is getting a lot of the risk and almost no economic benefits or jobs from this,” said Susan Christopherson, a professor at Cornell University’s Department of City and Regional Planning.’
  4. Speaking of crude, one reason why Putin is watching developments in the Ukraine is so many of Russia’s gas and oil pipelines pass through it.  [NYT: The Ukraine in Maps]
  5. Finally, air quality is so bad in China it is being compared to “nuclear winter”: sunlight can not get through to crops, potentially threatening the food supply.  [The Guardian: China’s toxic air pollution resembles nuclear winter, say scientists]