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Open research questions in Transportation Planning and Optimization

206 gap statements mined from Transportation Planning and Optimization papers in our 4.5M-paper local library, which holds 1,840 papers on the topic — drawn mostly from each paper's own stated research gap, future-work, challenge and limitation notes, and its abstract. The ones listed below are a selection still marked open; each names the study that raised it, with a DOI link where the paper has one.

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What the literature leaves open

  • The literature has focused on capital cities or objective financial data, leaving the perceptions of stakeholders in secondary urban hubs under-researched. There is a lack of empirical evidence on the structural determinants of perceived public transportation cost in Sub-Saharan African cities.

    Determinants of perceived urban public transportation costs in a rapidly urbanizing African City: evidence from Adama, Ethiopia · 2026 · DOI

  • The lack of knowledge about the uniqueness and continuity of equilibrium travel times. The need to study dynamic models rather than just static models.

    Continuity, Uniqueness, and Long-Term Behavior of Nash Flows over Time · 2026 · DOI

  • The evidence base on the nonlinear relationship between urban traffic congestion and rainfall is uneven. Existing studies predominantly focus on freeway systems, leaving urban road network dynamics underexplored. There is a need to understand how congestion responds to rainfall and affects metro ridership in urban areas.

    Nonlinear relationship of urban traffic congestion with rainfall and mediating effects on metro ridership: a comparison study of Guangzhou and Shenzhen · 2026 · DOI

  • Limited sensor data availability. The need to balance speed and reliability in route selection.

    Reliable Vehicle Routing Problem Using Traffic Sensors Augmented Information · 2025 · DOI

  • Uncertainty about new station development ridership may result in lower development revenues and transit operating burdens. The lack of a comprehensive evaluation method for assessing space efficiency in TOD metro station areas.

    Space Efficiency of Transit-Oriented Station Areas: A Case Study from a Complex Adaptive System Perspective · 2025 · DOI

  • The study faced the challenge of limited prior data on travel behavior in emerging cities of Ethiopia. The study had to develop a comprehensive survey to collect data on travel behavior and its relationships with individual and household attributes.

    Characterization of urban mobility behavior in emerging cities of Ethiopia: the case study of Bahir Dar using structural equation modeling · 2025 · DOI

  • The limited coverage of public transportation modes in Alexandria. The lack of a comprehensive and sustainable transportation network that considers both formal and informal transit. The need for a study that investigates the accessibility of public transportation modes and their interaction with informal transit.

    Unveiling transportation disparities: investigating accessibility gaps in metropolitan cities using GIS—a case study of Alexandria, Egypt · 2024 · DOI

  • Low Emission Zones (LEZs) are a central instrument of environmental policy across Europe, yet little is known about how public acceptability evolves as regulations are progressively reinforced.

    Another Brick in the Wall: Public Acceptability of Low Emission Zones Reinforcement · 2026 · DOI

  • While prior studies have extensively examined this relationship in the context of commuting and leisure travel, the role of non-emergency medical transportation (NEMT) service quality in shaping SWB remains underexplored.

    How do non-emergency medical transportation services influence patients’ subjective well-being? · 2026 · DOI

  • Rainfall increasingly stresses urban road networks, yet how congestion responds across the full range of rainfall intensity, and how that response propagates to rail transit, remains incompletely understood.

    Nonlinear relationship of urban traffic congestion with rainfall and mediating effects on metro ridership: a comparison study of Guangzhou and Shenzhen · 2026 · DOI

  • Generally, these cities lack data for detailed analysis, so transport planning based on modeling and simulation is difficult to implement.

    Reorganization of public transport systems in Global South cities and its relation with quality of life: A case study of Mexicali, Mexico · 2026 · DOI

  • However, the psychological processes through which crowding perception affects metro travel satisfaction have not been extensively explored in existing literature.

    Mechanisms influencing metro passenger satisfaction: A subjective crowding perception perspective · 2026 · DOI

  • Future research modelling local and city-wide impacts of all stages of the SRL are warranted.

    Developing a multimodal transport simulation model using a case study of the Suburban Rail Loop East, Melbourne, Australia · 2026 · DOI

  • These negative effects arose from a lack of stakeholder inclusion, limited knowledge of each other’s operational planning processes, and insufficient leadership across all organisations.

    Staggered school start times for reducing peak travel demand: A case study of implementation processes and stakeholder impacts · 2026 · DOI

  • However, it remains insufficiently understood which specific interior design aspects of modes of public transport contribute most to the MDGG.

    Regional rail interior design: assessing the Mobility Design Gender Gap (MDGG) in Germany · 2026 · DOI

  • Future research should explore preference heterogeneity within the agent-based framework.

    Evaluating the effectiveness and equity implications of car-use reduction policies through multi-agent simulation · 2026 · DOI

  • While this travel strategy has direct consequences for forecasting passenger flow distribution in congested networks, little is known about the travelling backwards phenomenon and why people adopt this travelling behaviour.

    Why do they travel backwards? Understanding passenger travel behaviour in congested urban public transport systems · 2025 · DOI

  • While the literature provides definitions of duplication and uses duplication indicators to analyze transit system performance, a comprehensive definition of the duplication concept is still lacking.

    A comprehensive analysis of duplication in public transportation · 2025 · DOI

  • Limited spatial coverage, inadequate feeder routes, overcrowding, and excessive travel times of the BRT system.

    Strategic Approaches to Alleviate Traffic Congestion and Enhance Urban Mobility in Peshawar · 2026 · DOI

  • Consider other aspects of user needs that require deeper analysis with more variables. Include other unofficial methods of transportation in the study. Investigate the impact of modern technologies on public transportation accessibility.

    Unveiling transportation disparities: investigating accessibility gaps in metropolitan cities using GIS—a case study of Alexandria, Egypt · 2024 · DOI

  • But the heterogeneity in the population for broader policy objectives, namely the prioritisation of improvements across transport modes, had rarely been examined in detail.

    Public preferences for transport infrastructure improvements: An analysis of attitudinal drivers · 2026 · DOI

  • The novelty of this study lies in its integrated decomposition framework, which jointly quantifies existing, toll-induced, and land-use-generated traffic components and links their long-term projections directly to vehicle-kilometers-traveled-based emission estimation, an integration rarely addressed in conventional transportation impact assessments.

    Trip Generation and Emission Estimation for a Toll Access-Integrated Economic Area: A Peri-Urban Corridor Case Study · 2026 · DOI

  • Compare the relative effectiveness of system-level reforms against incremental operational adjustments. Incorporate multi-source data or longitudinal designs to further mitigate common method variance and validate the robustness of these associations. Test and refine proposed policy interventions targeting structural levers through longitudinal study designs.

    Determinants of perceived urban public transportation costs in a rapidly urbanizing African City: evidence from Adama, Ethiopia · 2026 · DOI

  • Show that discrete versions of the model behave similarly to the continuous model as the packet size goes to zero, - Investigate the situation where users make approximately optimal route…

    Continuity, Uniqueness, and Long-Term Behavior of Nash Flows over Time · 2026 · DOI

  • Further studies can be conducted to investigate the impact of other factors on travel behavior, - Studies can be done to compare the travel behavior of different cities in Ethiopia, - Research can be conducted to develop more efficient and sustainable urban transport…

    Characterization of urban mobility behavior in emerging cities of Ethiopia: the case study of Bahir Dar using structural equation modeling · 2025 · DOI

Most-cited papers in Transportation Planning and Optimization

Most recent work

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Open questions raised by papers listed on this page include: (1) There is a lack of empirical evidence on the structural determinants of perceived public transportation cost in Sub-Saharan African cities (“Determinants of perceived urban public transportation costs in a rapidly…”, 2026); (2) The lack of knowledge about the uniqueness and continuity of equilibrium travel times (“Continuity, Uniqueness, and Long-Term Behavior of Nash Flows over Time”, 2026); (3) There is a need to understand how congestion responds to rainfall and affects metro ridership in urban areas (“Nonlinear relationship of urban traffic congestion with rainfall and mediating…”, 2026); (4) Low Emission Zones (LEZs) are a central instrument of environmental policy across Europe, yet little is known about how public acceptability evolves as regulations are progressively reinforced (“Another Brick in the Wall: Public Acceptability of Low Emission Zones…”, 2026); (5) While prior studies have extensively examined this relationship in the context of commuting and leisure travel, the role of non-emergency medical transportation (NEMT) service quality in shaping SWB remains underexplored (“How do non-emergency medical transportation services influence patients’…”, 2026). 206 gap statements have been mined from Transportation Planning and Optimization papers in our 4.5M-paper local library, which holds 1,840 papers on the topic; the gaps come from whichever of those papers state one. They are mostly the research gaps the authors state and the papers' abstracts, plus future-work, limitations and challenges passages. The ones listed below are a selection still marked open; each names the study that raised it, with a DOI link where the paper has one, so you can read the original claim in context.

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