A discrete MMAP for analysing the behaviour of a multi-state complex dynamic system subject to multiple events.

A complex multi-state system subject to different types of failures, repairable and/or nonrepairable, external shocks and preventive maintenance is modelled by considering a discrete Markovian arrival process with marked arrivals (D-MMAP). The internal performance of the system is composed of several degradation states partitioned into minor and major damage states according to the risk of failure. Random external events can produce failures throughout the system. If an external shock occurs, there may be an aggravation of the internal degradation, cumulative external damage or extreme external failure. The internal performance and the cumulative external damage are observed by random inspection. If major degradation is observed, the unit goes to the repair facility for preventive maintenance. If a repairable failure occurs then the system goes to corrective repair with different time distributions depending on the failure state. Time distributions for corrective repair and preventive maintenance depend on the failure state. Rewards and costs depending on the state at which the device failed or was inspected are introduced. The system is modelled and several measures of interest are built into transient and stationary regimes. A preventive maintenance policy is shown to determine the effectiveness of preventive maintenance and the optimum state of internal and cumulative external damage at which preventive maintenance should be taken into account. A numerical example is presented, revealing the efficacy of the model. Correlations between the numbers of different events over time and in non-overlapping intervals are calculated. The results are expressed in algorithmic-matrix form and are implemented computationally with Matlab.Junta de Andalucía, Spain, under the grant FQM307Ministerio de Economía y Competitividad, España, MTM2017-88708-PEuropean Regional Development Fund (ERDF

The Cascade Algorithm for Finding all Shortest Distances in a Directed Graph

Matrix methods for finding shortest distances are convenient and efficient when the lengths of shortest paths are wanted between all pairs of vertices in a graph. The Cascade algorithm requires substantially fewer operations to find these distances than does the standard matrix method.

Rethinking the taxi. Case study of Hamburg on the prospects of urban fleets for enhancing sustainable mobility

Despite being a vital part of the urban mobility system, the taxi receives little attention from planners and policy makers; thus, its potential contributions to enhance sustainable mobility are often overlooked. Consequently, this paper adopts an innovative perspective to rethink the taxi for enhancing sustainable urban mobility. It takes a closer look at the prospects of urban taxi fleets for supporting the transition of a city’s mobility system towards sustainability. The work is attributed to the project “Future Urban Taxi” under the initiative “Ambient Mobility Lab” supported with funding from the Ministry of Economic Affairs, Labor and Housing of the federal state Baden-Württemberg in Germany. To relate to a specific urban context, ten German cities were previously analyzed and Hamburg chosen as a use case. The city provides good availability of both taxi and mobility data, and the municipality is comparatively open to innovative concepts (e.g., the voluntary introduction of the fiscal taximeter, allowing more flexible pricing). By analyzing the use case Hamburg, “Future Urban Taxi” focuses on two main challenges: (i) how the taxi as a vehicle has to adapt to user demand and specific urban contexts, and (ii) how the taxi as a system can be integrated into the mobility system of a city in a more effective and sustainable way. A qualitative methodology consisting of the collection and qualitative assessment of expert interviews, as well as a scenario and gap analysis, was used to assess the potential of various taxi concepts. Three future scenarios for the year 2025 were built around them to arrive at three taxi concepts—the electric taxi, autonomous taxi and shared taxis. Each of these can contribute to the sustainability of Hamburg’s urban mobility system in varying degrees. The highest contribution lies in the implementation of shared-taxi services. They are rather easy to implement and can achieve quick benefits both for the customers and taxi operators. The electrification of taxis is rated second, since it requires investments in infrastructure and new forms of operations of the vehicles. The autonomous-taxi concept is least likely to be implemented soon, even though it could offer quite a few benefits. The reason is that there are still a lot of uncertainties (technical, spatial, legal) regarding this technology. By highlighting the need to rethink the taxi, this paper offers an insightful understanding of this mobility service in the specific urban system of the case study city of Hamburg