Evaluating Urban Transportation Accessibility: A Systematic Review of Access Dimensions and Indicators

In the realm of sustainable urban planning and land use, accessibility is of paramount importance, necessitating a judicious balance between the immediacies of mobility needs and the overarching tenets of sustainability. Research consistently underscores the primacy of accessibility as an objective within planning frameworks [1]. Accessibility should be construed not merely as a facet of transportation but as an encompassing construct that integrates various elements vital for in-depth analysis.

This study is predicated on the objective of delineating the interrelationship between accessibility and urban services. Through a comprehensive literature review, it endeavors to deepen the understanding of accessibility and to propel its conceptual development forward.

Accessibility's ultimate aim is the amelioration of urban environments to facilitate complete and equitable utilization by individuals of all abilities. Consequently, contemporary analysis must extend beyond transportation infrastructures to embrace a broader spectrum of contributory elements. The literature reveals a plethora of definitions and methodologies for measuring accessibility indicators, reflecting the complexity and multifaceted nature of the concept.

The influential analysis by Geurs and Van Wee [2] represents a significant portion of the literature that dissects the various components of accessibility and its attributes of interpretability, commonly cited across scholarly articles. Given that an intricate comprehension of accessibility's true essence owing to its technical and multifaceted nature has the potential to drive societal transformation, this research is committed to a methodical exploration of the nexus between accessibility and urban services. Employing a systematic literature review alongside a network bibliometric analysis has been instrumental in pursuing this aim.

The article is systematically divided into several sections for clarity and coherence: Section 2 introduces the general concept of accessibility, laying the groundwork for subsequent discourse. Section 3 categorizes accessibility criteria, informed by the analytical groundwork of Geurs and Van Wee [2]. Section 4 expounds on the methodological approach underpinning the article analysis. Lastly, Section 5 consolidates and discusses the findings, culminating in a set of conclusions that advance the current understanding of accessibility within urban environments.

Accessibility is a concept that has played a central role in physical planning over the past 50 years. Improving access is a goal that has now made its way into major transportation planning and transport policymaking worldwide. Hansen [3], in his classic explanation about accessibility and land use, provides the first actual description that accessibility is the ease of reaching desired destinations and is increasingly used as a planning model for displacement. Accessibility enhances performance and activities in specific locations and is a spatial and social phenomenon that influences mobility. By prioritizing mobility, proper access can be provided, ensuring that people with different abilities can fully utilize the urban environment [4].

In the literature, access criteria have been suggested as a primary tool for gaining a proper understanding of access. Various indicators of accessibility, encompassing theoretical, operational, interpretability, and communication aspects, offer avenues for their utilization in achieving evaluation objectives.

Given that accessibility is defined and measured using different indicators, this section outlines the main approaches based on the literature review:

• Location and Land Use Criteria: The first discussed index is the location-based index, which involves the spatial distribution of different land uses, quantified in terms of quantity (residential density, employment) and quality (employment level, housing value, service importance). Measures based on this index provide insights into locations and are commonly used by policymakers to assess land use and transportation comprehensively at the regional and local levels.

This index can be generalized into the following important set for examining corresponding actions:

1. Gravity-based Criterion: This criterion measures travel opportunities and incorporates costs discounted based on travel time or distance. It focuses on improving accessibility to destinations and emphasizes the spatial distribution of origins and destinations in relation to land use. The gravity-based criterion is widely accepted in the transportation field due to its strong theoretical foundation and superiority over cumulative opportunity measures [5].

2. Cumulative opportunities: This criterion estimates opportunities within a specific range and threshold of attraction-based travel costs. It is essential for clarifying budget allocation debates and investigating people's differential access to various travel methods. However, a widely accepted limitation of this criterion in politics is its failure to consider the effect of competition for available opportunities [6].

3. Travel Mode Criterion: This measure, called travel mode, is derived from the location-based index. It focuses on the predominant modes of travel, namely private cars, public transport, and active modes (walking, cycling). It is commonly used in transportation choice studies. It is worth noting that studies comparing access levels by car and transit using location-based indicators provide relatively simple estimates of mode-specific travel time.

4. Proximity-based criterion: This index is based on the proximity of key destinations, such as city cores or transit stations, and is particularly associated with walking in many places.

5. Travel time criterion: There is a strong positive relationship between this criterion and access. Other access variables within the location-based index subgroup are estimated to be marginally significant. Travel time calculations may also incorporate a mode index, typically relying on available data from transportation models and programs.

• Transportation Component: This criterion assesses the effectiveness of the transportation system in bridging the distance between origin and destination and the specific transportation method used. It encompasses criteria based on infrastructure measures and the environmental dimension.

1. Criterion Based on Infrastructure Measures: This criterion provides insights into the performance or service level of transport infrastructure, with a focus on the quality of the transport network.

2. Criterion based on the Environmental Dimension: it focuses on energy consumption and its external effects.

• Time-based Component: This component considers time-related constraints, such as the availability of opportunities at different times of the day and the time individuals have for specific activities. Other time components, including arrival and departure time, waiting time, and total travel time, can be included in this index through multiple estimates [6].

• Individual Level and Social Component: This component considers individual facilities, personal limitations, and socioeconomic characteristics to assess access. Research indicates that people perceive access levels differently based on individual indicators, and their willingness to travel to access opportunities varies. Consequently, this access index introduces as relativity, which can lead to biased results when considering the absolute parameters of well-being and sustainability.

The analytical method used in this research is the combination of systematic literature review and a bibliometric analysis approach in terms of quality and combination of results. It provides a deeper understanding of the content of the analyzed issue. On the other hand, this approach is a comprehensive bibliometric analysis, and the development of the subject over time is done using quantitative analysis of publications and their bibliographic features. In bibliometric analysis, publication data such as title, authors, keywords, summary, sources and citations are used as parameters.

In this study, network analysis has also been done using VOS Viewer software. In visual network analysis, nodes represent publications. Its dimension, the number of citations and simultaneously the arcs provide the relationship between two or more publications according to their bibliographic sources.

Searching for articles in scientific databases was considered until 2022. In the first stage of this process, keywords such as access, criteria, time and access, land use, etc., were used to search the titles and abstracts of publications. In the second stage, filters such as the number of citations (an average of 10 citations per year of publication) and DOI identifier (guarantee of authenticity) were considered. Finally, for the qualitative evaluation of the study and considering the purpose of the research, publications from 2010 with more complete coverage of the topic under discussion were selected.

The selected publications cover the period from 2010-2022. At the beginning of this search process, 184 articles were obtained from Google Scholar and Scopus databases. In the following, 114 publications were selected by applying restrictions in order to achieve a more favourable result.

In continuing this process, considering the indicators proposed by Geurs and Van Wee [2], discussed in the 3rd part of this study, we classified the selected publications and collected information extracted from them in line with the discussed approach. Finally, descriptive analysis was performed by VOS viewer software.

As mentioned earlier, according to the literature, access criteria are divided into several components and subsets. The selected articles are shown as a bibliographic network according to Figure 1.

Accessibility measures usually demand greater data and resources compared to similar mobility measures. It is crucial we take advantage of emerging data sources like Global Positioning System data for precise tracking of passenger movements, social media data for inferring travel preferences and goals, transport field specifications, and automatic passenger counters. These new and innovative data sources play a vital role in obtaining accurate information.

Literature review shows in order to consider many dimensions of access, in addition to the criteria, more attention should be paid to the reliability, cost, passenger characteristics (such as taste, justifications and restrictions), safety, travel aesthetics and destinations. One of the main limitations stated for the accessibility criteria is that these only consider a specific goal of the trip and a specific time to make the trip or the individual's state. It is worth mentioning that some research considers walking distance or time required for access and exit to transportation, but pedestrian and bicycle access part of other modes mentioned about Private Mobility Vehicle (PMV) usually are not considered in studies. In addition, researchers often use the distance of a person's location to the usual walking transportation for basic prediction and evaluation of transportation-related economic development and also consider individual characteristics in this issue. The obtained results show that young people and men are more inclined to devote time to walking to reach the transportation station than the elderly and women, respectively. On the other hand, car ownership, household income, and household size also have important effects on access. Households with more vehicles, higher income, and more members are more likely to limit access to certain mode choices.