Light Rail Transit Line Research Articles

Predicting transit ridership using an agent-based modeling approach

Accurate ridership estimation is pivotal in the advancement of sustainable transit systems, be it for proposed or existing transit networks. A multitude of methods, including travel demand models, direct ridership models, and regression models, have been employed by practitioners and researchers to estimate ridership at both station and network levels. However, travel demand models, frequently utilized for new transit lines, exhibit intrinsic limitations due to their aggregate nature and complexity based on their types. Researchers have also identified deficiencies, such as the incapacity to capture small spatial resolutions and specific station characteristics, as these models are predominantly designed for large-scale analyses.This study aims to overcome these limitations by introducing a novel approach that utilizes three microscopic agent-based models to develop a travel demand modeling suite, providing a policy-sensitive forecasting tool. The suite comprises three agent-based models: SILO-MITO-MATSim. Validation of the model against previous year data is conducted, and projections are made for future years. The model is applied to estimate network-level ridership for the proposed ‘Purple Line,’ a light rail transit line planned by MDOT, MTA, Maryland, which will integrate with the Washington D.C. Metro, the fourth largest transit system in the USA, boasting an average daily ridership of half a million. The study’s findings indicate an anticipated ridership of approximately 31,230 passengers in the inaugural year of 2027. The proposed model offers a robust and policy-sensitive solution empowering decision-makers to make informed choices to support a sustainable transportation system.

The HWAM-EMD-GRU Forecasting Model for Short-Term Passenger Flow in an Airport Light Rail Transit Line

Accurate forecasting of airport light rail transit line (ALRTL) outbound passenger flow is critical to the optimal operations of both light rail and airport systems. Considering the nonlinearity, non-stationarity, uncertainty, and periodicity of outbound passenger flow in the ALRTL, we propose a combined forecasting model that integrates the Holt and Winters additive model (HWAM), empirical mode decomposition (EMD) and gated recurrent unit (GRU). Firstly, the edge effect of EMD will greatly affect the performance of the forecasting model. To overcome this, we extend the passenger flow by HWAM. After that, the decomposition method, EMD, can be applied to passenger flow, and several intrinsic mode function (IMF) components can be extracted. After extracting all the IMFs, the remaining part is referred to as the residual (Res) component. Then, a correlation test is performed on all the components, followed by their aggregation. Finally, the GRU is used to predict each of the aggregated components, and the prediction of aggregated components requires reconstruction. To verify the performance of the HWAM-EMD-GRU, we conducted a comparative study on the hourly passenger flow data for Beijing Daxing International Airport Express and set the autoregressive integrated moving average model, HWAM, Prophet, and GRU as the baseline. Predictions of the HWAM-EMD-GRU combined model demonstrated higher accuracy than baseline models, with a root mean square error of 83.52 (Prophet is 110.21) and mean absolute percentage error of 8.32% (Prophet is 12.48 %). The experimental result shows that the HWAM-EMD-GRU forecasting model offers more accurate predictions.

Causal identification of transit-induced property value uplift in Canada's Waterloo Region: A spatio-temporal difference-in-differences method application

Renewed interest in light-rail transit (LRT) in North America has heightened the need for an improved understanding of transit impacts on land value uplift (LVU). A number of studies have investigated the relationship, with findings varying with local contexts and estimation methods. Most of these studies focus on the aggregate effects of transit using cross-sectional models, but do not examine the spatial and temporal heterogeneity in transit impacts through quasi-experimental approaches. To bridge this gap, we build a set of spatio-temporal difference-in-differences (STDID) models for the causal identification of transit-induced land-value uplift, taking the new LRT line in Canada's Waterloo Region as the case study. The study contributes to the transit-induced LVU literature in several ways. First, we account for the space-time influence of recent comparable sales in price determination, a transaction data-generating process often excluded in hedonic studies. Second, our models reveal the disaggregate effects of transit policies in different station areas and transit phases. Third, we provide a pre-LRT analysis in a car-dependent mid-sized urban area, which offers insight into speculative investment in TOD areas and/or resident anticipation of future accessibility benefits. The findings can provide guidance for value-capture programs and cost-benefit analysis for transit-oriented development in mid-sized cities.

Planning for greenery under Light Rail Transit (LRT) in the section of Kelapa Gading North Jakarta

The high mobility of the population in Jakarta encourages the creation of mass transportation modes that can carry a large number of passengers at one time. One of them is the construction of LRT transportation that uses an elevated track. Due to limited land, the pillar of the elevated track was built in the ROW of the existing road. The case of Kelapa Gading LRT was the pillar of the LRT built in the median Boulevard Raya Road. This elevated tract produces an elongated space under the rail track. The space under the LRT track line has the potential to construct a green lane because the currently existing conditions were not planned well. So the purpose of this research was to develop a green lane plan under the LRT rail track by carrying out the concept of a green corridor which be able to maintain the quality of the environment, and able to improve visual quality, and can be beneficial for the community in accordance with the concept of sustainable development. In the proposed plan, the vegetation applied to the road sections such as the traffic island roundabout, U-Turn, and the space under the LRT line combines the use of vegetation to maintain environmental qualities, identify vegetation, plant which shows the road direction, and climate amelioration vegetation. Green technology planning is also proposed at the research site by applying a litter trap, raingarden, porous oncrete, eco-drainage, and solar power system for street lighting.

Why Did the “Missing Middle” Miss the Train? An Actors-In-Systems Exploration of Barriers to Intensified Family Housing in Waterloo Region, Canada

(1) Background: Missing Middle (MM) housing may be critical to address decreasing housing affordability and to achieve critical density in transit-oriented neighborhoods; however, its production is in decline. We report on a case study of housing development around a new light-rail transit line in the Region of Waterloo, Canada, investigating the puzzle of how a residential building boom coincided with decreasing housing affordability. (2) Methods: Following participatory co-creation and communication of background research characterizing housing demand with stakeholder partners, we created a data narrative arguing that MM housing was desired by residents and profitable for developers and then used it to guide semi-structured interviews with planners and real estate industry stakeholders. Based on these interviews, we developed a qualitative system map and causal loop diagrams that demonstrate interactions between key actors (residents, brokers, planners, developers, and investors) as mediated by boundedly rational real estate demand expectations. (3) Results: Our interviews identify multi-faceted barriers, beyond demand perception, to MM housing development. Systems analysis illustrates how high-density, small-unit high-rise development can become locked in, concurrently locking out MM housing. (4) Conclusions: Our research identifies barriers to MM housing supply by articulating the systemic feedbacks between the planning and land/housing market realms and reveals key leverage points, empowering planners to develop policies that catalyze hoped-for housing market supply responses to increase housing affordability. Based on these findings, we suggest targeted interventions: multi-unit base residential zoning, MM site plan typologies, non-profit and co-op financing, unit-mix requirements, pre-build MM condo purchase by municipalities or non-profits, and MM demonstration projects.

Quantifying the Impact of Light Rail Transit on Commercial Property Values: New Evidence From Greater Kuala Lumpur, Malaysia

Evidence shows that improving accessibility through investment in public transport, such as urban rail transit, can have a positive impact on land values and subsequently on property values. This paper, therefore, presents empirical results on the impact of proximity to the nearest Kelana Jaya light rail transit (LRT) station on commercial property values in Greater Kuala Lumpur, Malaysia. Using the hedonic pricing model, the paper attempts to quantify these effects. The sample consists of 399 office transaction data for the years 2014 and 2018, marking more than a decade of Kelana Jaya LRT line operation in the region. The results show that an office unit located within a 400 m radius of the nearest Kelana Jaya LRT station enjoys the highest premium, reaching about 0.637%, which, at the mean, is MYR802,620 (USD200,655). However, the premium appears to decrease rapidly for an office unit located within a radius of 401–800 m from the nearest Kelana Jaya LRT station, reaching 0.232% (MYR292,320 or USD73,080 of the mean office price). From the results of this study, it can be concluded that this paper contributes to the growing literature on the positive relationship between urban rail transit and commercial property values. More importantly, this paper provides the necessary evidence for the potential implementation of a land value capture policy as an alternative source of revenue to fund or partially fund urban rail transit in Greater Kuala Lumpur, which is presently absent.

Demographic Analysis, Patron Satisfaction, and Suggestions within the Light Rail Transit (Lrt) Line 2 Service: A Scholarly-Based Service Enhancement Matrix (IMRAD)

Demographic Analysis, Patron Satisfaction, and Suggestions within the Light Rail Transit (Lrt) Line 2 Service: A Scholarly-Based Service Enhancement Matrix (IMRAD)

A Literature Review on Demographic Analysis, Patron Satisfaction, and Suggestions within the Light Rail Transit (LRT) Line 2 Service: A Scholarly-Based Service Enhancement Matrix

A Literature Review on Demographic Analysis, Patron Satisfaction, and Suggestions within the Light Rail Transit (LRT) Line 2 Service: A Scholarly-Based Service Enhancement Matrix

Mapping displacement through lived experiences: Countermapping transit-induced gentrification in Kitchener-Waterloo, Ontario

This article seeks to enhance our understanding of the role transit infrastructure plays in shaping patterns of gentrification and displacement by directly engaging with residents living along a new light rail transit (LRT) line who are affected by these processes. Displacement is difficult, if not impossible to statistically measure, yet much of the literature on transit-induced gentrification relies on quantitative analysis. Our approach is based on a collaboration between academic and non-academic partners that assembles, analyses and amplifies the experiences of marginalised residents in order to shift planning, policy and political conversations about the nature of change within our region. Through interviews with approximately fifty low-income residents living along a new LRT corridor in Waterloo Region, we analyse patterns, processes and experiences of displacement and the loss of already existing affordable housing. This information has been mapped to produce narratives that counter the dominant views which have celebrated an urban revival along the LRT corridor. Our research emphasises the need to incorporate lived experiences of displacement into central positions within the debates that make and shape cities.

Risk Analysis for Train Collisions Using Fault Tree Analysis: Case Study of the Hanoi Urban Mass Rapid Transit

The urban mass rapid transit (UMRT) Line HN2A is the first light rail transit line in Vietnam. It is also the first time the operational safety assessment for the whole life cycle of a railway project is applied and assessed by an applicable scientific tool. While various industry standard methods have been deployed in many countries, their application is not appropriate for assessing the outdated railway infrastructure in Vietnam. This article proposes a method for generating safety risk models for train collisions using the fault tree analysis (FTA) technique. The FTA method comprehensively evaluates the fundamental error and failure probability that could potentially lead to accidents in general and train collisions in particular on Line HN2A. The study describes the procedure for establishing FTA and determining assumptions based on technical specifications and similar railway systems. Compared with the statistical failure results using data from operational tests and commissioning (2018–2021) of the metro line, the results here indicate that this is a reasonable theoretical model applicable to UMRT in Vietnam. The theoretical model will be processed to generate the first-ever scientific risk assessment system based on empirical evidence. In addition, real-time accident and operation data will continue to be collected and compared to the theoretical model to improve its accuracy. The findings of this study could serve as a starting point for risk management on current and future freight, passenger, and metro lines in Vietnam.

Transit justice: community perceptions and anticipations of a new light rail transit line in Prince George’s County, Maryland, United States

ABSTRACT Transit-oriented developments are increasingly important as they address many modern urban environment problems such as urban sprawl, pollution, and rising rates of non-communicable diseases. However, their implementation is difficult as cities are complex systems. Urban health improvements is a labyrinthian process as enhancements in one area often leads to several unintended side effects, such as transit-induced gentrification or injustice. The Purple Line Impact on Neighborhood, Health and Transit (PLIGHT) study examines the community impact of a forthcoming light rail train line in Prince George’s County, Maryland, a predominantly Black and Hispanic community. Results show that despite mindful planning the Purple light rail train is subject to ‘Transit Orientated Development’ idealism, which can propagate transit injustices. This qualitative study highlights the disconnect between this idealism and practice. It explicitly identifies the most vulnerable of transit users and highlights how neoliberal urbanism contributes to health inequities in Transit Orientated Development practice.

Impact of a light rail transit line on physical activity: Findings from the longitudinal Travel Assessment and Community (TRAC) study

IntroductionIncreasing transit infrastructure could increase transit use and result in higher physical activity if users actively travel to access transit. Few studies have rigorously examined transit use and physical activity change from before to years after among residents living close versus farther away from new transit options. MethodsAn initial sample (n = 722) of residents living either close (<1 mile network distance; exposed) or farther away (>1 mile network distance; unexposed) from future new light rail transit (LRT) stops in the Seattle/King County area were recruited and assessed prior to LRT opening and again 1-2 and 3–4 years later. At each assessment timepoint, residents wore an accelerometer and GPS data logger for 7 days and completed a 7-day travel log and demographic and attitudinal survey. Difference-in-difference analyses examined longitudinal change between those exposed versus unexposed to LRT in physical activity, walking (both utilitarian and recreational), and transit-related walking, and transit use. ResultsThere was no differential change by LRT exposure in overall physical activity (including or not including light intensity physical activity), recreational walking, or utilitarian walking, with most decreasing significantly in both exposure conditions through follow-ups. There was a differential change in transit-related walking, with those exposed to LRT slightly increasing such physical activity to the most distal follow-up, but the difference from the unexposed condition was modest (<2 min/day). There was no substantial differential change over time in transit use by LRT exposure. ConclusionsExposure to a new light rail line did not markedly change the frequency of transit use of nearby residents, but did result in a small increase in transit-related walking relative to those unexposed. This did not differentially change the amount of overall physical activity or time spent walking compared to residents living farther away from the new LRT.

Making a Successful LRT-Based Regional Transit System: Lessons from Five New Start Cities

This paper examines five metropolitan areas where light rail transit (LRT) lines serve as regional transit backbones. The paper defines a successful LRT-based regional transit system as one with high riding habit and productivity for all combined modes in each metropolitan area, and as also having high LRT ridership and productivity. Based on these criteria, Portland emerges as a successful LRT-based regional transit system. Our analysis reveals three characteristics that explain the Portland transit system’s strong performance: the network’s dispersed nature, the overlay of a higher-speed, high-frequency regional LRT network atop the local bus system, and the use of transfers to provide passengers easy access to a diverse array of destinations. We examine the performance of all five metropolitan areas with respect to these characteristics using a combination of agency data and insights from interviews with key informants.

Effects of Light-Rail Transit on Traffic in a Travel Corridor

An important debate is taking place over the value of transit in easing traffic congestion. This study sought to quantify the effect of light rail transit (LRT) on traffic in a travel corridor and provide quantitative data that can be used to shape future transportation policies aimed at reducing traffic congestion, energy consumption, and air pollution. Using a quasi-experiment design and data before and after the University of Utah’s TRAX LRT line was opened, we estimated that traffic on the street with LRT (400/500 South) decreased by 7,500 to 21,700 due to the availability of a high-quality transit serving destinations along the line, and, most important, the University of Utah. Traffic on 400/500 South decreased despite significant development in the corridor and expansion of the university. Based on our estimates, LRT along 400/500 South saves about 362,000 gallons of gasoline and prevents about 7 million pounds of CO2 from being emitted each year.

The TROLLEY Study: assessing travel, health, and equity impacts of a new light rail transit investment during the COVID-19 pandemic

BackgroundThe COVID-19 pandemic disrupted life in extraordinary ways impacting health and daily mobility. Public transit provides a strategy to improve individual and population health through increased active travel and reduced vehicle dependency, while ensuring equitable access to jobs, healthcare, education, and mitigating climate change. However, health safety concerns during the COVID-19 pandemic eroded ridership, which could have longstanding negative consequences. Research is needed to understand how mobility and health change as the pandemic recedes and how transit investments impact health and equity outcomes.MethodsThe TROLLEY (TRansit Opportunities for HeaLth, Livability, Exercise and EquitY) study will prospectively investigate a diverse cohort of university employees after the opening of a new light rail transit (LRT) line and the easing of campus COVID-19 restrictions. Participants are current staff who live either < 1 mile, 1–2 miles, or > 2 miles from LRT, with equal distribution across economic and racial/ethnic strata. The primary aim is to assess change in physical activity, travel mode, and vehicle miles travelled using accelerometer and GPS devices. Equity outcomes include household transportation and health-related expenditures. Change in health outcomes, including depressive symptoms, stress, quality of life, body mass index and behavior change constructs related to transit use will be assessed via self-report. Pre-pandemic variables will be retrospectively collected. Participants will be measured at 3 times over 2 years of follow up. Longitudinal changes in outcomes will be assessed using multilevel mixed effects models. Analyses will evaluate whether proximity to LRT, sociodemographic, and environmental factors modify change in outcomes over time.DiscussionThe TROLLEY study will utilize rigorous methods to advance our understanding of health, well-being, and equity-oriented outcomes of new LRT infrastructure through the COVID-19 recovery period, in a sample of demographically diverse adult workers whose employment location is accessed by new transit. Results will inform land use, transportation and health investments, and workplace interventions. Findings have the potential to elevate LRT as a public health priority and provide insight on how to ensure public transit meets the needs of vulnerable users and is more resilient in the face of future health pandemics.Trial registrationThe TROLLEY study was registered at ClinicalTrials.gov (NCT04940481) June 17, 2021, and OSF Registries (https://doi.org/10.17605/OSF.IO/PGEHU) June 24, 2021, prior to participant enrollment.

Prevention Cost Components Priorities for Urban Rail Infrastructure Projects

The Mass Rapid Transit (MRT) and the Light Rail Transit (LRT) have been establishedas the largest transport infrastructure projects in Malaysia and cost over RM40 billion. Due to the vigorous development of both projects, accidents at project sites have causedfatalities, serious injuries,and damage to properties. Many accidents haveoccurred during the operation of rail. In Malaysia, the latest rail accident occurred on 24thMay 2021had injured200 people when two coaches onthe Kelana Jaya LRT line collidedin the capital Kuala Lumpur. Railaccidents create significant economic impacts on the country, stakeholders,and society. Studies on the prevention costs during the pre-construction stage of a project are scarceand have mainly focused on the financial losses or accident costs incurred by the contractor and victim.Amongthe limitations of the existing safety and health cost models is that they do not include the costs incurred by the client for the safety prevention activities. Therefore, this research aims to study the types of prevention cost components that must be the priorities for urban rail infrastructureprojects. The data analysed in this study wasobtained from a total of eight (8) projects involved in the MRT 1 (SBK Line) and ten (10) projects involved in the MRT 2 (SPP Line). The prevention cost components priorities were determined by the coefficient of variation (CV) percentage. The finding shows ten (10) prevention cost components,and nineteen (19) prevention cost items wereallocatedduring the pre-construction phase. The cost components werethe Plant Costs, OSH Management Costs, Housekeeping Costs, Special Work Condition Costs, Evaluation and Monitoring Costs, Emergency Response Costs, Electrical Work Costs, PPE and Training Costs, Facilities on Site Costs, and Insurance Costs. This study finds out that all these prevention cost components are important to prevent accidents for the employees when carrying out work on-site. Therefore,clientsshould considerthe availability of these cost componentsin the planning and preparation phasesof an urban rail infrastructure project.Thus, the most logical amount of prevention cost components can be achieved in the future project and creating the awareness of these costs to the clients.

Effects of an urban light rail line on health care utilization and cost: A pre-post assessment

In September 2015, a new light rail transit (LRT) line opened in metro Portland, Oregon, USA. We used this natural experiment to conduct an interrupted time series analysis of the effects of LRT introduction on health care costs. We hypothesized that such costs would decline over time based on demonstrated health benefits of increased transit-related physical activity. We wanted to evaluate whether introducing new LRT would lower the trajectory of monthly health care costs generated by a large sample of members of an integrated health care plan living near a new LRT station (cases) relative to similar members who did not (controls). We hypothesized that new LRT would stimulate higher transit use and consequently, higher transit-related physical activity among plan members living relatively closer to the LRT. However, the new LRT did not lower health care costs as hypothesized, at least relative to prior costs and within the first 4.5 years after the LRT opening. Results were consistent across outpatient visit costs and medication costs, as well as when only the 10 percent most expensive health plan members were considered. Although we did not find evidence of transit-related reductions in health care costs, research into the potential influence of public transit on physical activity and downstream health care costs remains warranted. Mechanisms through which such influence manifests are complex. Our negative results based on extensive and objectively measured health care cost data will contribute to the accuracy and validity of future models of the societal benefits of public transit.

Assessing the impact of powerplant fuel type and trip modal shift on pollutant emissions of mashhad light rail transit

The Light Rail Transit (LRT) Line 1 of Mashhad city was constructed in 2011 as a new transportation mode to meet the increasing demand for urban trips and reduce the traffic congestion. In addition, due to city air pollution, another objective of using LRT as public transportation is to reduce pollutants emissions. The positive impacts of development of rail system on reducing air pollution on an urban scale is undeniable, but in a global perspective, reports about its effect on pollutants emissions are limited and contradictory. The main objective of this study is to quantitatively determine the emission reduction potential of Light Rail Transit of Mashhad city, considering the effect of powerplants fuel type and trip modal shift to rail system. In this study, a field survey is conducted on change in transportation mode and then the amount of pollutants emissions is estimated over a 9-year period. The results of survey study show that 91.83% of the LRT trips used to be by automobile, taxi, bus and motorcycle, and 8.16% new trips are created by LRT. Transportation modal shift leads to decrease in CO, HC and CO2 emissions while the emissions of NOX, SO2 and PM increase. Accordingly, 18.32% decrease in CO2 emission is obtained which is the main reason of global warming. The trend of pollutants emissions from 2011 to 2019 reveals that increasing the number of LRT trips and decreasing the electricity generation emission factor are two main factor that improves the performance of LRT. The decrease in electricity emission factor is due to change in fuel of powerplants from mazut and diesel to natural gas. It is estimated that increasing the number of LRT trips and decreasing the electricity emissions factor by 20% compared to 2019 respectively leads to 101.4% and 67.8% CO2 emissions savings due to modal shift. Finally, it is concluded that expanding Mashhad LRT system will have a significant impact on pollutants emissions reduction.

Measuring transit-oriented development in Algerian light rail transit lines by using hybrid multi-criteria decision making and open data sources

Selecting transit-oriented development (TOD) sites along light rail transit lines (LRTLs) is difficult for city planners and decision makers. Selecting the actors and the charges involved in the prioritising process, particularly, when selecting between regions with criteria, is critical. Therefore, this study provided a measuring tool to identify TOD opportunities between regions, and outlined its defining criteria through an easy, rapid, and cost-effective method. In this measuring tool, a multi-criteria decision making approach with a hybrid weighting technique was used for ranking eight criteria representing five principles of TOD. Subsequently, simple additive weighting and technique for order of preference by similarity to ideal solution methods were used to rank LRTLs. Furthermore, open data sources were used for data acquisition, and QGIS software was used for data analysis. The application of this method on six LRTLs of Algeria revealed that three tramlines are highly appropriate for TOD implementation, and the rest of the three LRTLs require interventions to some TOD requirements. The density criterion was confirmed as the critical parameter of TOD measurements around tramlines in Algeria followed by exchange poles and transit ridership. The station number and ticket price were the least critical criteria. The results revealed that integrated weighting is critical for investigating TOD as a new planning approach. This result promoted the assignment of accurate weights, which can be useful for decision makers when working in unexplored fields or during indecisive situations. Weighting criteria can be used as guidelines for the major requirements for locations suitable for TOD. Eventually, these results are valuable for future investments of TOD projects along LRTLs or other transit modes.

Causal evaluation of the health effects of light rail line: A natural experiment

Background and objectiveNo research to date has causally linked built environment data with health care costs derived from clinically assessed health outcomes within the framework of longitudinal intervention design. This study examined the impact of light rail transit (LRT) line intervention on health care costs after controlling for mode-specific objectively assessed moderate-to-vigorous physical activity (MVPA), participant-level neighborhood environmental measures, demographics, attitudinal predispositions, and residential choices. Data and methodsBased on a natural experiment related to a new LRT line in Portland – 282 individuals divided into treatment and control groups were prospectively followed during the pre- and post-intervention periods. For each individual, we harness high-resolution data on Electronic Medical Record (EMR) based health care costs, mode-specific MVPA, survey-based travel behavior, attitudinal/perception information, and objectively assessed built environment measures. Simulation-assisted longitudinal grouped random parameter models are developed to gain more accurate insights into the effects of LRT line intervention. ResultsRegarding the “average effect” of the LRT line intervention, no statistically significant reductions in health care costs were observed for the treated individuals over time. However, substantial heterogeneity was observed not only in the magnitude of effects but its direction as well after controlling for the within- and between-individual variations. For a subgroup of treated individuals, the LRT line opening decreased health care costs over time relative to the control group. Further comparative analysis based on the findings of heterogeneity-based models revealed that the effect of LRT intervention for the treated individuals differed by individual characteristics, attitudes/perceptions, and neighborhood level environmental features. ConclusionsThe study revealed the presence of significant effect modifiers and distinct subgroup structures in the data related to the effects of LRT line intervention on health care costs. Severe implications of ignoring unobserved heterogeneity are highlighted. Limitations and potential avenues for future research are discussed.