New publication: Miller, H.J., Jaegal, Y. and Raubal, M., “Measuring the geometric and semantic similarity of space-time prisms using temporal signatures,” Annals of the American Association of Geographers (online first).
Well-established techniques exist for measuring the similarity of space–time paths. These measures support clustering and aggregation of space–time paths as well as moving objects database queries based on similar movement patterns or semantics. Little attention has been paid, however, to the analogous problem of measuring space–time prism (STP) similarity, despite comparable applications. This article presents and evaluates a method for measuring STP similarity through dimensionality reduction that leverages their inherent temporal ordering. The technique sweeps an STP along the time axis and derives one-dimensional temporal signatures based on a measured STP property that captures its geometry or semantics. These temporal signatures can be visualized directly as curves. We can also apply existing space–time path similarity measures to these signatures. To demonstrate the feasibility of this approach, we perform two sets of experiments measuring geometric and semantic similarity among STPs and assess the information within these curves using visualization, Fréchet distances, and clustering techniques. Results suggest that the temporal signature curves capture meaningful similarities and differences among STPs.
Existen técnicas bien probadas para medir la similitud de las trayectorias espacio–tiempo. Estas medidas soportan agrupamiento y agregación de trayectorias espacio–tiempo lo mismo que consultas de bases de datos de objetos en desplazamiento con base en patrones de movimiento o semántica similares. Sin embargo, poca es la atención que se ha deparado al problema análogo de medir la similitud del prisma espacio–tiempo (STP), a pesar de aplicaciones comparables. Este artículo presenta y evalúa un método para medir la similitud del STP mediante la reducción de dimensionalidad que apalanca su inherente ordenamiento temporal. La técnica barre un STP a lo largo del eje del tiempo y deriva firmas temporales unidimensionales basadas en una propiedad STP medida, que captura su geometría o su semántica. Estas firmas temporales pueden visualizarse directamente como curvas. También podemos aplicar a estas firmas las mediciones existentes de similitud de las trayectorias espacio–tiempo. Para demostrar la viabilidad de este enfoque, realizamos dos conjuntos de experimentos midiendo la similitud geométrica y semántica entre STPs y evaluamos la información dentro de estas curvas usando visualización, distancias Fréchet y técnicas de agrupamiento. Los resultados sugieren que las curvas de firma temporal capturan similitudes significativas y diferencias entre los STPs.
New publication: Lee, J. and Miller, H.J. (2019) “Analyzing collective accessibility using average space-time prisms,” Transportation Research Part D: Transport and Environment, 69, 250-264.
Abstract: The space-time prism (STP) is the envelope of all possible travel paths in space and time between two anchor locations and times, measuring accessibility for an individual given a designated travel and activity episode. Although the STP provides a powerful measure of individual accessibility, transportation researchers often need to analyze accessibility at collective-levels for planning and policy analysis. Deriving a representative STP of a set of individual STPs would provide a general idea of how collective members’ accessibility is performing. However, there is no analytical time geographic method to calculate a collective-level representative STP that is consistent with individual STPs. To fill this gap, this research develops the concept of average space-time prism (ASTP). The ASTP is a representative STP of a group of individual STPs with respect to size, shape, and location. We develop methods for calculating an ASTP using analytical time geography and elliptic Fourier shape analysis techniques. The ASTP provides a geometric and visual summary of collective accessibility: it can be used to generate representative STPs for aggregate geographic units such as neighborhoods and cities based on individual-level data. A possible application of the ASTP is the spatial equity analysis of accessibility. The ASTP can be located at individuals’ anchor locations and overlaid with opportunities, enabling in-situ comparisons between individual versus collective accessibility and accessibility equity analysis considering geographic contexts. We illustrate this ASTP’s capability when measuring the impacts of new transit service on healthcare access equity in a neighborhood in Columbus, Ohio, USA.
Lee, J. and Miller, H. J. (2018) “Measuring the impacts of new public transit services on space-time accessibility: An analysis of transit system redesign and new bus rapid transit in Columbus, Ohio, USA,” Applied Geography, 93, 47-63.
- Lack of access to opportunities contributes to poor social and health outcomes.
- Columbus, OH introduced a transit route and schedule redesign and bus rapid transit.
- We analyze impacts on accessibility to opportunities in a deprived neighborhood.
- Detailed route and schedule data allow high resolution accessibility analysis.
- The new bus rapid transit has a much greater impact on accessibility
The absence of effective access to opportunities and services is a key contributor to poor socio-economic and health outcomes in underserved neighborhoods in many cities. The city of Columbus, Ohio, USA is attempting to enhance residents’ accessibility by providing new public transit services. These new services include a major Transit System Redesign (TSR) of the conventional bus network and the introduction of a new bus rapid transit, named CMAX. Using a high-resolution space-time accessibility measure, we analyze whether these new public transit services will change residents’ accessibility to job and healthcare in an underserved neighborhood of Columbus. Also, we assess whether enhancing the CMAX service to reduce delays (e.g., reserved lane, off-board payment system) will improve accessibility. The high-resolution space-time accessibility measure in this study uses published public transit schedules via the General Transit Feed Specification (GTFS). We use multiple departure times during a day to account for the temporal fluctuations of accessibility based on the transit schedule changes. We also consider the operating hours of job opportunities and healthcare services. Results suggest that the TSR yields ambiguous benefits for accessibility to jobs and healthcare. However, the new CMAX service and its potential upgrades lead to a substantial increase in both job and healthcare accessibility. The results can be used for city officials and urban planners to evaluate the effectiveness of public transit innovations in improving accessibility.
Keywords: Transportation; Space-time accessibility; Public transit; Bus rapid transit; Jobs; Healthcare