Τhe Mobility-as-a-Service (MaaS) concept has been developed within the past ten years and integrates public and private mobility modes to promote on-demand mobility ^[1][2][1,2]. The International Association of Public Transport (UITP) defines MaaS [3]: “MaaS is the integration of, and access to, different transport services (such as public transport, ride-sharing, car-sharing, bike-sharing, scooter-sharing, taxi, car rental, ride-hailing and so on) in one single digital mobility offer, with active mobility and an efficient public transport system as its basis. This tailormade service suggests the most suitable solutions based on the user’s travel needs. MaaS is available anytime and offers integrated planning, booking and payment, as well as en route information to provide easy mobility and enable life without having to own a car”.

Recent studies have highlighted the role of MaaS in increasing efficiency both for mobility systems and individual users [4]. MaaS aims to redesign the future of urban mobility by integrating multi-modal transportation with Information and Communications Technologies (ICT) and app-based technologies ^[5][6][5,6]. MaaS provides a more convenient and more sustainable solution than owning and driving private cars, which subsequently leads to the reduction of congestion in city centers and suburbs, the reduction of traffic accidents, and the reduction of required space for parking [7]. The assumption that supports MaaS is that a seamless integration of a wide range of mobility services, such as bike-sharing with public transportation services, or carpooling with public transport [8], is more appealing than owning, maintaining, and operating a private vehicle ^[4][7][4,7].

Despite the fact that several MaaS solutions have been developed lately at global level, the MaaS concept has its roots in the Nordic nations. It is claimed that Heikkilä coined the term in 2014 [9]; her thesis resulted in a call for pilot projects from the Finnish Innovation Agency and led to the founding of the first MaaS company in 2015, named MaaS Global [2]. They developed the Whim app that operated in Finland, Austria, Belgium, Japan, and the UK [7]. Moreover, around the same time in Sweden, a MaaS trial known as Go:Smart (later renamed UbiGo) was financed by the Swedish Innovation Agency to develop, test, and evaluate ways of offering a mobility solution for sustainable traveling in the city of Gothenburg. UbiGo was launched in Stockholm in 2019 but ceased operation in early 2021 [7]. Although several pilots have been demonstrated and evaluated, only a small number of them resulted in a product in the market.

The MaaS provides the ability for travelers to combine public and private transport modes within a city or beyond by using a single application. A key success factor for the MaaS is the utilization of a reliable account to book and pay for used transport services ^[10][13]. MaaS is often referred to as a tool that could help increase the sustainability of transport systems ^{[11][12][13][14]}[14,15,16,17]; however, a universal definition of MaaS has not yet been established ^[15][18].

2. MaaS Impacts

The anticipated MaaS benefits focus on the optimization of the existing public transport services within a city and the increase in travelers’ satisfaction ^[16][11]. MaaS may also improve network efficiency by optimizing supply and demand, especially during peak hours when certain modes/routes are under-utilized, and reduce traffic congestion, transport costs for end-users, and car ownership ^[16][11]. The implementation of MaaS may lead to emission reductions ^[17][19] and offers seamless end-to-end mobility to its users ^[18][20]. If Maas is structured and priced properly, it could provide benefits, including increased public transport ridership and active transport usage ^[19][21], and offer intermodal solutions ^[19][21] Soteropoulos et al. (2021) ^[20][22] mentioned MaaS as being one of the potential solutions for addressing the challenges of future mobility. They suggested that automated vehicles and MaaS could potentially reduce the need for private car ownership and provide more efficient and sustainable transportation options. Existing research indicates that MaaS contributes to reduced dependence on private vehicles ^[21][23], reduction of transport-related emissions ^[22][24], enhancement of transport system reliability ^[23][25], increased convenience and accessibility, reduced traffic congestion, and cost savings for users, increased flexibility for its users in the post-pandemic era ^[24][26], and new opportunities for innovation and new business models for involved stakeholders ^[25][27].

3. Planning and Implementation

The need to switch from single-mode planning to multimodality and build a resilient transport network has been also highlighted by the recent COVID-19 pandemic. The emphasis on resilience implies going beyond single-mode resilience to cross-modal, systemic resilience optimization ^[26][28]. However, there is a high degree of ambiguity surrounding the MaaS concept, planning, and implementation ^[27][29]. Following a critical literature review by Jittrapirom et al. ^[27][29] the identified core characteristics when implementing a MaaS are the integration of transport modes, tariff options, the platform, coordination of multiple actors, use of technologies, demand orientation, registration requirements, personalization, and customization. MaaS has been studied in the literature both theoretically ^[11][28][14,30] and practically. At the practical level, MaaS has been tested either within the framework of research projects ^[29][30][31][31,32,33] or has been implemented in several urban areas (e.g., Ubigo and Whim app). Table 1 presents MaaS applications that have been tested or released in the market. ^][37]. Reasons for users not joining Ubigo included fare affordability (i.e., more expensive than the existing transport solution), a perceived mismatch between the user and the service, and lack of infrastructure (e.g., bike-sharing or car-sharing stations) to serve new users ^[35][10]. The substantial barriers to implementing MaaS were also identified and grouped into categories for two European metropolitan areas, Budapest (Hungary) and Manchester (UK) ^[16][11]; Table 3 presents the main ones. MaaS utilize apps that offer a monthly subscription or a pay-as-you-go service for a single or a group of travelers to combine transport modes and use them with a single payment ^[10][13]; different apps and platforms have been deployed to support local MaaS systems. MaaS Global released Whim, the first MaaS solution in the world ^[38][39]. Currently, Whim operates in Helsinki and Turku (Finland), Antwerpen (Belgium), Vienna (Austria), West Midlands (UK), multiple cities in Switzerland, and Greater Tokyo (Japan). The Whim is an award-winning mobility app that facilitates mobility by offering two types of MaaS service: purchasing a season ticket (predefined mobility packages), and a single trip ticket (pay-as-you-go). The season ticket includes unlimited usage of public transport, taxi, city bikes, car rental, e-scooters, and shared bikes. In Los Angeles and Denver (US), a mobility platform was launched in 2016 to assist residents and tourists make travel choices more easily ^[10][13]. The “Go Denver” and “Go LA” apps estimated different routes, including the greenest one, by considering individuals’ destinations and desired arrival time. The apps aggregated and calculated the time, cost, carbon footprint, and health benefits of walking, biking, driving, parking, public transit, and emerging ride-hailing options ^[39][40]. The UbiGo app in Gothenburg, Sweden ^[30][36][37][32,37,38] offers a monthly subscription for public transport, car sharing, car rentals, bike sharing, and taxi services ^[35][10]. At the moment, there are at least three ongoing MaaS initiatives in the city of Madrid, yet there is no collaboration among them. The main challenge that they face is also confirmed by literature findings: the lack of a governance framework for MaaS ^[40][41]. Given the different circumstances and conditions in different cities and regions, it seems unlikely that a single MaaS model would be universally applicable ^[41][42]. Boero et al. ^[30][32] described the MaaS concept and its implementation in the context of the IMOVE project. Furthermore, they described the organizational and technological enablers for MaaS and the main objectives and elements in the participating pilot sites. IMOVE was first implemented in four European areas (Living Labs) including Göteborg, the Västra Götaland region, the Berlin Brandenburg region, Greater Manchester, and Turin. The living labs combined several modes to deploy MaaS services, such as public transport, car-pool, bike-pool, taxi, U-Bahn (subway), tram, bus, free-floating and stationary vehicle-sharing, scooter-sharing, bike-sharing, and Uber.

4. Challenges and Barriers

Several stakeholders, including transportation planners, operators, and policy makers are interested in planning and implementing MaaS ^[16][11]. However, potential societal, operational, financial, and regulatory barriers might hinder the MaaS success as concluded in the literature ^[16][11]. For example, dependence on the MaaS to improve mobility and accessibility of individuals may create equity issues ^[42][43], which should be considered within the transport policy and practice field. Furthermore, without a supportive built environment and high-quality public transport system, MaaS will likely not succeed to change travelers’ behavior ^[26][28]. The degree these challenges are addressed affect the degree the potential benefits of MaaS are achieved ^[43][44]. EU-funded projects have recently demonstrated MaaS solutions in several cities (Table 2), yet the challenges that they faced during the implementation process are mainly technological integration of different Transport Service Provider (TSP) platforms and institutional issues. Table 2 presents a summary of recently demonstrated pilots in EU projects and the main challenges they faced during testing. The MaaS4EU project concluded that major legal and regulatory barriers exist for the implementation of MaaS services ^[44][45] that make participating in a MaaS scheme difficult for suppliers and public service providers. The project stresses that regulations and passenger rights can largely differ across different modes, due to the lack of a unimodal approach in the EU legislative framework. The MyCorridor project ^[45][46] categorized barriers as well as enablers into five categories: User and market, Technology, Organization, Business, and Legal. However, legal issues for MaaS implementation were emphasized by further grouping them into: data protection, cybersecurity, intellectual agreements, consumer and payment laws, data interoperability, and local regulations. One of the latest MaaS demonstrations, within the framework of the Shift2Maas project ^[25][27] highlighted challenges related to regulations, including data privacy (GDPR) as well as the local variation of regulations. Shift2MaaS proposed a roadmap, summarizing recommendations based on three pillars: regulation, business models, and technical issues. The IP4MaaS project extracts information and lessons learned from the MyCorridor and Shift2MaaS projects to build use cases and plan its demonstration. Except for major challenges that interested stakeholders may face during MaaS implementation, barriers may also exist on the users’ side. Sochor et al. ^[35][10] conducted a six-month field test in Gothenburg, Sweden to explore motivations and barriers to adopting new travel services. Potential users were initially motivated by curiosity, convenience, and fare savings. Results showed that is vital to generate interest and excitement in potential users regarding a new transportation scheme. Users provided positive feedback, but service providers faced regulatory and institutional barriers ^[36