Citation: Biehle, T. Social
Sustainable Urban Air Mobility
in Europe. Sustainability 2022,14,
9312. https://doi.org/10.3390/
su14159312
Academic Editors: Vittorio Di Vito,
Gabriella Duca and Raffaella Russo
Received: 27 June 2022
Accepted: 26 July 2022
Published: 29 July 2022
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sustainability
Review
Social Sustainable Urban Air Mobility in Europe
Tobias Biehle
Department of Work, Technology and Participation, Technische Universität Berlin, Marchstraße. 23, MAR 1-1,
Abstract:
The first step to steer passenger Urban Air Mobility (pUAM) towards the necessity of
sustainability is to understand its impact on our urban transportation systems. This research empha-
sises the social footprint of passenger drones in scheduled operation as an early business model in
European Functional Urban Areas. The literature review is guided by the corresponding Sustainable
Urban Mobility Indicators (SUMI). The prospective impact which the introduction of pUAM has on
the evaluation of European transportation systems regarding their affordability for the public, their
inclusivity for mobility-impaired groups, their accessibility to commuters and the level of customer
satisfaction is analysed. Furthermore, the impact of pUAM on the perceived quality of public urban
space is examined. Results indicate the overall social footprint of passenger drones in European
transport systems to be negative. Early market pUAM may lead to an unbalanced distribution of
potential benefits, with services tailored to address only a limited number of citizens. Highlighting
pathways for a societal benefiting technology, recommendations are provided for urban planning
and city development.
Keywords:
passenger UAM; urban planning; vertiports; affordability; inclusivity; accessibility;
acceptance; satisfaction; SUMI
1. Introduction
In 2016, official sources estimated around 10,000 electric aircrafts would be in operation
for the transportation of passengers in European urban airspaces by 2050 [
1
]. Today,
investments in the respective technologies and regulatory frameworks have led to a more
favourable outlook in market research. While first services are expected to launch in
2024, a broader market take-off is projected one year later, and about 160,000 vehicles are
predicted to be in commercial operation by 2050 [
2
]. Meanwhile, efforts are being made
to construct a digitalised and highly automated system for urban air traffic management
(UTM/U-Space) to allow for the efficient and safe integration of these vehicles into our
build environments [
3
]. From a mobility rationale, it is claimed that passenger Urban Air
Mobility (pUAM) will reduce travel time with its integration in intermodal transportation
networks, will lighten traffic congestion on the ground due to mode shifting into the air
and will ultimately contribute to more sustainable transportation compared to ground-
based alternatives due to the use of electric energy sources [
4
]. Moreover, it is argued
that pUAM will not become an exclusive mode-choice for the few, but will soon become
affordable, inclusive and accessible to the broader public, satisfying the transportation
needs of common people and adding to the overall quality of life in our cities [5].
However, the vision of sustainable urban (air) mobility will not materialise by it-
self [
6
]. Initially, a contingency on sustainability effects regarding the use of drones for
passenger transportation must be expected. This may include undesirable impacts on
travel behaviour, e.g., increasing travel distances and, along with it, a renunciation of more
sustainable ground transportation [
7
]. What is more, the introduction of low-level air traffic
in conjunction with necessary transport infrastructure may increase social and welfare dis-
parities among citizens [
8
]. To anticipate such planning difficulties, an ongoing technology
assessment of pUAM becomes highly relevant in (European) transportation research. After
Sustainability 2022,14, 9312. https://doi.org/10.3390/su14159312 https://www.mdpi.com/journal/sustainability
Sustainability 2022,14, 9312 2 of 17
all, it forms the precondition for urban planning authorities to make confident decisions on
this new technological opportunity in cooperation with industry and communities.
This content analysis provides the audience with a systematic literature review on the
social dimensions of sustainable transportation as depicted by the respective European
Sustainable Urban Mobility Indicators (SUMI). To apply this framework, pUAM will be
considered as a private mobility service complementary to public urban transportation
systems. The expected impact of pUAM on the overall affordability, inclusivity and
accessibility rating of urban transportation systems will be analysed. Further, the expected
impact of pUAM on citizens’ perceived satisfaction with the transportation system as well
as on the perceived quality of public urban space is investigated. For the analysis, the
criteria and aspects underlying the original SUMI are adopted on the specifics of pUAM.
To further facilitate the analysis, the conceptual understanding of Functional Urban Areas
(FUA) is applied. These comprises densely inhabited cities and their less densely populated
commuter catchment area. Consequently, an FUA does not necessarily correspond to the
administrative borders of a municipality or region [
9
]. In Europe, FUAs can be characterised
by typically polycentric spatial structures and functionally linked areas, a minimum level of
social and economic diversity, the existence of public spaces and greenery and a minimum
level of public services, including the provision of public transport [10].
Business cases that are explicitly considered in the analysis are inner-city commutes
and pUAM as linkage between city and periphery, e.g., satellite cities, suburban or rural
areas [
11
]. Any service provision is thereby dependent on dedicated ground infrastructure
for passenger access and egress, so called vertiports. The vehicles are considered piloted,
battery-powered and able to vertically take-off and land (eVTOL) with two to five pas-
sengers on board. Booking will be conducted by digital means as part of the Mobility as
a Service (MaaS) environment [
12
]. As it is expected for early market operation, pUAM
services in this analysis are assumed to be scheduled operations between limited numbers
of attractive, urbanised locations [13]. However, on-demand operation will be considered
as part of the discussion and outlook. Within this analytical framework, substantiated
prospects can help to assess the impact that an introduction of pUAM services will pose
for the social sustainability assessment of European urban transportation systems. The
findings may help authorities and planners to reflect a suitable role for pUAM in urban
development and to steer a potential technology implementation towards the most vital
target of sustainable mobility. What is more, the findings may contribute methodologically
to a further adoption of SUMI towards new forms of aerial transportation.
2. Social Sustainability in Urban (Air) Transportation
In respect to the long tradition of transportation research, sustainable mobility is
a relatively young concept which took off with the 1992 Green Paper of the European
Commission “The Impact of Transport on the Environment: A Community strategy for
“sustainable Mobility” [
14
]. The document acknowledged an increasingly problematic
relationship between transport’s positive effects on economic welfare and its negative
environmental impacts. From there onwards, research and policy foci, methodological
approaches as well as research questions have undergone substantial changes [
15
]. The
ongoing observations regarding the impact of transportation on economy and society as
well as their inter-relatedness have led to more integrated, interdisciplinary perspectives.
To describe this complexity and to illustrate trade-offs or synergies in the context of
political decisions-making and urban planning, a large number of authors refer to the triad
of ecological, economic and social pillars of sustainable mobility (e.g., [
16
–
18
]). While the
economic pillar emphasises the role of mobility to ensure resource efficient production
and development, the ambition of environmentally sustainable mobility contributes to the
preservation of our climate, the conservation of non-renewable resources, the protection of
biodiversity and the abatement of air, water and ground pollution. The social pillar ensures
that mobility contributes to community cohesion by supporting equity, participation, health
and security in society [
19
]. Socially sustainable mobility systems would therefore ensure
Sustainability 2022,14, 9312 3 of 17
that everyone is able to satisfy his or her transportation needs to engage in social and
economic life on an equal basis. Therefore, the affordability of transportation for everyone
is highly relevant, as well as its spatial accessibility and its inclusivity, e.g., for mobility-
impaired groups [20].
Sustainable Urban Mobility Plans (SUMPs) offer the possibility to anchor these long-
term goals for integrated freight and passenger transport, increased quality of urban life
and environmental protection in transportation planning processes. SUMPs have been
proposed by the European Commission as part of the “Action Plan on Urban Mobility” in
2009. In 2014, corresponding community guidelines have been approved by the European
Union General Directorate for Mobility and Transport. Since 2019, in a revised edition, these
guidelines constitute a fundamental methodological reference for municipal stakeholder
initiatives to foster sustainable urban mobility in Europe and abroad [
19
]. SUMPs envisage
to: (1) define a future vision and milestones for; (2) assess the current performance of;
(3) implement measures in; and (4) re-evaluate an urban transportation system [21].
Sustainable Urban Mobility Indicators (SUMI) thereby reflect the conceptual under-
standing of sustainable urban transport in European policy and are the methodology to
assess the actual impact of sustainable urban mobility planning practices described above.
The set of these altogether 19 indicators is used to: (1) describe the performance of an
overall urban transportation system or a certain aspect in a standardised form; (2) identify
strengths and weaknesses in respect to certain policy targets or indicator thresholds; as well
as to (3) assess the effectiveness of implemented policies and practices, e.g., by analysing a
shift before and after the introduction of new means of transportation. The social dimension
of sustainable urban transportation is reflected within five indicators, which measure the
affordability of public transport for the poorest group, the inclusivity of public transport for
mobility-impaired groups, the accessibility to mobility services for citizens, the satisfaction
with public transport as well as with the quality of public spaces [21,22].
3. Methodology
This study utilises these SUMI to assess pUAM on its prospective impact on trans-
portation systems in European FUAs. The methodological approach is to review the current
literature to understand the prospective positive and negative implications of a pUAM
introduction on the relevant indicators and, with this, on the overall social sustainability
rating of urban transportation systems. It is to note, however, that the applicability and
integrity of the indicators of pUAM characteristics have not yet been tested. In addition,
while conventional air and water transportation is excluded from an assessment of a city’s
transportation system via SUMI, authors do call for the indicators to be revised and adopted
with regard to the impact assessment of emerging transport technologies that stem from
the electrification, automatisation and digitalisation of urban mobility [
23
]. To this end, this
article makes its contribution.
3.1. Adoption of SUMI on pUAM Characteristics
Preliminary work through a specific framework for the evaluation pUAM stems from
al Haddad et al. [
24
]. The authors suggest a list of Key Performance Indicators (KPIs) to
assess the environmental, socio-economic and transport potential of pUAM in the FUAs
of Upper Bavaria, Germany. To do so, a multi-criteria decision analysis was preformed
whereby experts weighted indicators in terms of relevance and measurability. A final
selection was made using a threshold method. In order to assess the validity of the selected
KPIs, they compared them with the SUMI. On the social dimensions of sustainable urban
transportation, mutually supported are the affordability or equity indicators and inclusivity
indicator. Exclusive to SUMI remain the access to the mobility services indicator, the
satisfaction with the transport service indicator and the impact on the quality of public
spaces indicator. On the other hand, the quality of life/welfare indicator and the privacy
disturbance indicator are proposed from the authors [
24
]. As both dimensions address
specific characteristics of pUAM, they are going to be reflected in the following adoption of
Sustainability 2022,14, 9312 4 of 17
the original SUMI framework as recognized by the Directorate-General for Mobility and
Transport of the European Commission. For indicator definitions see [25].
Affordability of public transport for the poorest group indicator:
The indicator
recognizes that public transport should be affordable for all parts of society in order to
make them equally available in social and economic life. It is originally measured by the
share of the poorest quartile of the population’s household budget required to use public
transport. In the context of pUAM, the indicator is particularly relevant as concerns exists
that future services will only address high-income households or business travellers [
26
].
Hence, the expected cost structure and future price development of pUAM is reviewed
from the literature in relation to average household incomes.
Inclusivity for mobility-impaired groups:
The standard of this indicator is to ensure
that people with reduced mobility can actively and fully participate in society rather than
experience discrimination and accessibility restrictions on public transport due to their
condition. From the original indicator, persons with reduced mobility can be quoted as
“those with visual and audial impairments and those with physical restrictions, such as
pregnant women, users of wheelchairs and mobility devices, the elderly, parents and
caregivers using buggies and people with temporary injuries”. Including people with
intellectual disability or impairment in this definition, the issue of reduced mobility is
prevalent for around 87 million people in the European Union alone [
27
]. Passenger
UAM-related services should therefore not create new barriers for people with disabilities
but allow them to enjoy the benefits of this innovation on an equal basis to customers
without impairments.
Access to mobility services indicator:
All neighbourhoods in a FUA should be mean-
ingfully connected to public transport in order for people to equally participate in social
and economic life. The indicator synthesises both averages, of how much distance people
have to bridge in order to reach public transport services and how often these services
are provided for the respective locality. Threshold values were defined for this in SUMI,
e.g., in a metropolitan area, poor accessibility would be accorded if people need less than
10 min to reach their local railway, but only if this station is served less than four times an
hour. Thus, the assessment of whether pUAM services meaningfully increase the access
to urban transportation systems will depends foremost on the spatial distribution of ver-
tiports, their reachability for customers within the respective catchment areas and their
operational performance.
Satisfaction with public transport indicator:
The satisfaction with public transport
or a particular mode of transportation affects the actual usage. That in turn affects the
potential to stimulate economic growth, social and territorial cohesion as well as positive
environmental effects emerging from the public transportation system. According to SUMI,
satisfaction can be measured by evaluating citizens’ perceptions towards the affordability,
safety, reliability and easiness to obtain a particular mode of transport. Hence, the more
pUAM meets the expectations of the general public in these dimensions, the higher the
satisfaction. Since services are not yet in place to evaluate actual user experiences, mainly
results of acceptance research will be reviewed, in particular on the willingness to use and
pay for pUAM.
Quality of public spaces indicator:
The quality of public spaces affects mobility
behaviour and urban life quality. Designed to analyse results from the European Commis-
sion’s Urban Audit, the indicator pools the satisfaction of local populations with public
spaces such as pedestrian areas and green spaces such as parks. In a broader understanding,
public spaces may be evaluated by their openness, by the physical and environmental relief
and the welcoming sensory perceptions they provide, as well as by their vibrancy of safety
and control that citizens feel when engaging in these places (cf. [
28
]). Regarding UAM, a
changing perception of these qualities due to UAM vertiports as well as a novel degree
of low-level air traffic will be considered in the analysis, including privacy and welfare
implications as suggested by al Haddad et al. [
24
]. Results from acceptance research and
the planning-related literature will be reviewed.
Sustainability 2022,14, 9312 5 of 17
3.2. Literature Analysis
To derive prospects for the social sustainability of early pUAM services, a literature-
based content analysis was conducted [
29
]. The relevant documents have been selected
from two databases: Web of Science and Google Scholar. To structure the selection process,
a list of English keywords was created [
30
]. Those were initially derived from the content
dimensions covered by the SUMI and substantiated by pUAM-specific keywords provided
from the KPI description by al Haddad et al. [
24
]. Boolean operators were used to refine the
search. From the initial pool of articles found via the search engines, additional documents
were identified via cross-reference searches [
31
,
32
]. Table 1provides an overview on
the adopted definitions of SUMI for pUAM and the deployed keywords to structure the
document research.
Table 1. Summary on the adopted SUMI framework for pUAM and keyword search.
SUMI Adopted for pUAM: Research Focus: Keywords for Database Search:
Affordability of pUAM for the poorest
Budget required to use pUAM on a
regular basis for commuting and
inner-city travel
uam AND affordability OR equity OR
operating costs OR pricing OR demand
Inclusivity of pUAM for
mobility-impaired groups
Prospective accessibility of pUAM
services and infrastructure to persons
with reduced mobility
uam AND inclusivity OR accessibility
OR equality
Accessibility of pUAM services
Spatial distribution of vertiports and their
performance.
uam OR vertiports AND accessibility OR
scalability OR modal share OR location
OR distribution OR capacity OR
performance OR passenger handling
Satisfaction with pUAM
Perceived satisfaction of using pUAM,
especially regarding its safety,
affordability, reliability and easiness to
obtain/convenience
uam AND user adoption OR acceptance
OR satisfaction OR reliability OR
affordability OR safety OR convenience
Impact of UAM on the quality of
public spaces
Impact of pUAM and related
infrastructure on the perceived
satisfaction with public spaces and on the
quality of urban life/welfare.
uam OR vertiports AND visual pollution
OR privacy OR public spaces OR urban
quality OR acceptance
Forty publications have been selected. Besides technical papers from academic and
public agencies such as the European Union Aviation Safety Agency (EASA) and National
Aeronautics and Space Administration (NASA), only peer-reviewed articles and, where
necessary, conference papers were considered. In regard to the constant gain of knowledge
in the field and to extend the scope of former reviews [
4
,
33
], this analysis focused on
publications from the year 2020 onwards. Excluded from this guideline are key sources,
e.g., the literature in fields with low publication density. The analysis was performed using
the qualitative content analysis software Atlas.ti (version 9), which allows the management
of larger data sets [
34
]. Codes were created deductively from the five indicators. The
collected codes in each analysis category were then summarized on content level [35].
4. Results
On the basis of the literature analysis, prospects for the implementation of pUAM
and its expected impact on the relevant indicator for the social sustainability of urban
transportation systems in Europe is presented.
4.1. Affordability of pUAM
Studies attempting to estimate the price of pUAM suggest it to be below the per-
kilometre price of existing helicopter point-to-point services but also far above the price of
traditional taxi services [
36
,
37
]. More precise and comparable examples have been found
only for eVTOLs with the capacity for three passengers, including one pilot. Based on a
calculation made in the U.S. context, the median price per passenger and mile was found
Sustainability 2022,14, 9312 6 of 17
to be USD 7 net [
38
], corresponding to approximately EUR 3.78 per kilometre in 2021.
Consequently, a 70 km flight from San Francisco to San Jose would cost around EUR 265 per
passenger. A comparable study on the use of pUAM to supplement public transportation in
the metropolitan FUA of Munich, Germany anticipates a price per passenger and kilometre
of EUR 4.94. The charge for a 70-kilometre regional flight from Munich to the city of
Ingolstadt would therefore be at least EUR 346 per passenger. For shorter inner-city trips
or suburban connections, the study proposes additional basic fares up to EUR 20, making
a 10-kilometre trip cost around EUR 70 per passenger [
39
]. The realism of these price
calculations is in respect to the multitude of deduced and sometimes daring assumptions,
e.g., regarding public funding of pUAM infrastructure, is hard to assess. Both examples
underline, however, that early pUAM cannot compete with common modes of urban
transportation (a monthly ticket for the extended Munich region Q1 2022 costs around EUR
230), let alone be affordable for regular commuting by broader parts of European society.
4.2. Inclusivity of pUAM for Mobility-Impaired Groups
The only reference found that specifically analyses the requirements of mobility-
impaired groups for the design of pUAM services is published by NASA as a technical
memorandum. It includes design considerations for the accessibility of ground infrastruc-
ture, vehicle access and cabin layout, consideration for in-flight operation and emergency
response, as well as for the accessibility of digitally mediated information, ordering, book-
ing and payment processes. The author emphasises the relevance of including the needs of
mobility-impaired groups from the earliest stages onwards into the design and develop-
ment process “of the overall system-of-systems network inherent in the UAM concept” to
create path dependencies in favour of an inclusive transportation service [40] (p. 6).
If and to what extent current private sector development is anticipating this appraisal
cannot be assessed from the literature. However, authors in the field of traditional avi-
ation stress that handling passengers with special needs is posing additional cost that
affects profitability and competition between airlines. This is particularly the case when
closely timed operational processes are ‘disrupted’, or when customers are entitled free
of charge to be guided by assistance personnel or to cargo space for mobility aids and
medical equipment [
41
]. Accordingly, for pUAM services, trade-offs between the degree of
inclusivity that could be offered and the financial requirements for infrastructure, vehicles
and adopted operational procedures needed to realise it must be acknowledged.
In future, legal obligations might assist in shaping the inclusivity of pUAM services
for mobility-impaired groups. For example, Straubinger et al. discuss pUAM as part
of public transportation in Germany, implying an applicability of the National Public
Transport Act [
42
]. This regulation obliges MaaS providers such as taxi, ride hailing or
ride pooling companies with a fleet size from 20 vehicles to ensure at least 5% of their
fleet to be accessible for disabled persons. Drawing this analogy, pUAM providers may
become committed to certain inclusivity standards for their vehicles and infrastructure as
a prerequisite for an operational approval from the licensing authority in the respective
territory. Besides national level jurisdiction, European legislation may as well demand
inclusivity standards for pUAM in future, as is currently in place for international aviation
carriers, ships as well as rail transportation [
43
,
44
]. Finally, missing inclusivity standards
might impact the overall customer satisfaction and public perception of pUAM negatively,
hence pressuring manufactures and operations to adopt [13].
4.3. Access to pUAM Services
The impact of pUAM on the accessibility rating of urban public transportation systems
in Europe will largely depend on the layout of vertiport networks, its reachability for
customers and its performance.
Sustainability 2022,14, 9312 7 of 17
4.3.1. Vertiport Placement
Regarding vertiport placement, studies apply demand-driven approaches, aiming
to identify connections that will create reliable revenue in early market operation. The
spatial distribution of vertiports is thereby dependent on the expectable number of trips
between catchment areas in a city and the likelihood that people for these trips will
choose pUAM over existing alternatives (cf. [
44
]). Primarily, high demand stems from
agglomerations of commuters traveling between transportation hubs, residential and
business districts of a metropolitan area [
11
]. As those are characterised by a certain density
of transport infrastructure and saturation with public/private transport services, pUAM
will likely represent an additional, potentially more time efficient mobility offer rather than
filling accessibility gaps in urban transportation systems. While it would be through the
commissioning and operation of vertiports in less connected neighbourhoods and remote
suburbs that the establishment of pUAM would increase the transport accessibility rating
in the respective areas, low demand counteracts such line of thoughts [45].
4.3.2. Reachability of Vertiports
The reachability of vertiports, e.g., in walking distance (a reasonable walking distance
is quoted to be up to 2 miles/3.2 km [
37
]) presents a relevant factor for pUAM services to
realise overall travel time savings compared to competing means of ground transporta-
tion [
46
]. However, the extent to which good reachability will be archived in complex urban
environments is difficult to foresee. In the current state of research, vertiports are computed
rather ideally into predefined catchment areas while planning vertiports is suggested to
prove more challenging in real world scenarios [47–50].
As one particular planning constraint, the available space may prove a barrier. In
regard to this, EASA already published technical design specifics for vertiports on the
ground as well as on heights, such as business buildings and car parks for congested urban
areas. Thereby, the vehicle touchdown and take-off area (TLOF) is covered by a rectangular
funnel that widens towards the top. No obstacles may protrude into this volume for safety
reasons. In the considered reference model (Volume Type 1), the height of the funnel is
around 30 metres and the take-off and landing area is two times the diameter of the smallest
circle enclosing the respective VTOL aircraft, which may be about 400 m
2
in size [
51
]. Thus,
even when adding necessary facilities for passenger handling, the already iconic renderings
of small landing pads on high-rise rooftops for a pUAM touch-and-go configuration in
inner-city districts appears feasible. However, with the capacity for one vehicle only, these
pads are significantly limited in their customer throughput rates. When anticipating the
time for eVTOL landing and egress of three passengers, respectively for the boarding of
three passengers and eVTOL departure with five minutes (process times are derived from
Preis and Hornung [
52
]) each, 36 persons per hour could be serviced in scheduled operation
under the most idealistic conditions. In mobility-on-demand operation, whereby more
people want to land at inner-city vertiports in the morning or take off after work, higher
costs, negative environmental impacts and, after all, operational inefficiencies are expectable
from a repositioning of empty eVTOLs. Aiming for higher performance, Rimjha and Trani
assume a size of around 8000 m
2
for a vertiport with parking stalls for eight eVTOLs, the
necessary taxiing areas and one TLOF [
53
]. This equals to the size of a football field. Thus,
when guaranteeing certain baseline capacities, the search for well-located infrastructure
areas within walking distance can be expected to become significantly more difficult.
In addition to the availability and the financial feasibility of such spaces, the localisa-
tion and operation of vertiports is expected to become influenced by safety regulations as
well as regulations for the protection of residents and the environment from harmful im-
pact [
33
]. Similar to airport planning, research suggests that a reconciliation with public and
residential interests, e.g., regarding urban fauna or protection from emissions, may impact
administrative decisions on vertiport sizing and operations [
54
]. In this respect, residential
acceptance becomes another relevant dimension of consideration. Besides externalities on
neighbours from noise, also visual pollution, security concerns, privacy or increased traffic
Sustainability 2022,14, 9312 8 of 17
and congestions in the surrounding area may foster a rejection of vertiports in economically
attractive catchment areas [
55
] p. 88. Factoring in these aspects, authors point out that
operational requirements of vertiports in interchange to questions of residential acceptance
could be lower on private industrial and commercially used spaces [
42
]. What is more,
participatory planning approaches involving residents are suggested to help mitigate social
vertiport planning obstacles, supporting the placement of vertiports in closer proximity to
its potential beneficiaries [56].
4.3.3. Vertiport Frequencies
Last, to understand the impact of pUAM on the overall accessibility rate of an urban
transportation system is the frequency with which vertiports and thus customers will be
served by eVTOLs. Thereby, a vertiport must always be comprehended as a system bound
to the capacities of its surrounding urban airspace and, hence, the U-Space management
efficiency. However, the throughput capacity rate of a vertiport itself is primarily affected
by (a) the vehicle specifics, including time for vertiport approach and departure, (b) the
available size of the vertiport impacting the organising of ground operation and, of course,
(c) turnaround times of the vehicles involving passenger handling [
57
]. The authors Preis
and Hornung contribute to a better understanding on how these operational parameters
affect the average wait time for pUAM passengers. Using an agent-based simulation, the
authors conduct a sensitivity analysis that includes varying parameters regarding passenger
demand, vertiport layout (pads, gates and stands) and processing times for eVTOLs and
passengers. The results suggest that while each vertiport can handle a certain amount
of constant demand with which low passenger wait times and reliable performance are
conceivable, temporal peaks in demand have a significant impact on delay times (this may
be less consequential for pUAM in scheduled operation and fixed ticket contingent, but
significant for future on-demand operation and asymmetric arrival and departure requests).
Much stronger, however, because growth is exponential after a certain tipping point, is
the impact of increased processing time for vehicles and customers as well as a decreased
availability of landing pads and gates on the average passenger delay. Hence, unexpected
disruptions in vertiport operation or sudden airspace restrictions due to weather change or
emergency operations may result in major delays for passengers [52].
4.4. Satisfaction with pUAM
As no large-scale pUAM services are available yet, the actual customer satisfaction
cannot be assessed. However, studies on the willingness to hire or pay for pUAM once
services are available can contribute to a more detailed understanding of the prospective
satisfaction with pUAM. In summary, the general willingness to use air taxiing is low.
For example, a population representative survey with 1000 respondents from Germany
finds that only 18 per cent are open to use air taxis for their individual mobility [
58
] p. 6.
However, Winter et al. show that the willingness to fly in an eVTOL increases the more
this action is perceived as useful in a given situation [
59
] and al Haddad et al. show that
the willingness to use pUAM increases the more the respondents associate the use of this
service with a reduction in travel time [
60
]. Consequently, it is more comprehensible that a
representative study commissioned by EASA with 3690 participants from six European
metropolises concludes that, on average, 49 percent of respondents would at least try out
and pay more for an air taxi under the condition that the given trip would be done in
half the time compared to using a road taxi service [
55
] p. 62. Thus, the usefulness and
advantageousness over its alternatives will be a decisive factor for customer satisfaction
with pUAM.
4.4.1. Perceived Safety
In respect to the perceived safety, the before mentioned surveys from EASA shows that
safety is rated the most prevailing concern for respondents from Europe [
55
] p. 73 while the
respective survey from Germany shows that 53% of respondents disagree on the question
Sustainability 2022,14, 9312 9 of 17
if they would consider passenger transport with air taxis to be safe [
58
] p. 9. For the
prospective satisfaction with pUAM services, this may be consequential. Lim et al. argue
that a high safety perception and trust in eVTOLs will be most important for a positive
user evaluation, especially for the initial stages of pUAM (priorities are expected to change
in favor of service orientation once pUAM services proof their reliability) [
61
]. Adding
to these results, statistical research suggest that respondents’ feelings of safety towards
eVTOLs strongly depend on how it is piloted. Chancey and Politowicz show in their study
design that the willingness to use remotely piloted pUAM services is lower compared to
services with an on-board pilot, as the latter is trusted more [
62
]. Similar results can be
found regarding the future potential for fully automated [
60
] or autonomous [
59
] eVTOL
operation in passenger transportation respectively. Authors indicate that the willingness
to use and pay for pUAM decreases the lower the level of respondents’ understanding
towards the technology responsible for flight control [
59
,
60
]. Comparable results have
been suggested in relation to automated long-haul aircrafts [
63
]. Thus, a safety perception
towards the technology is somewhat a precondition to feel satisfied with pUAM. However,
research suggests that trust levels or safety perceptions are significantly influenced by
certain demographics. For example, it is suggested that younger persons have a higher
affinity to vehicle automation while older persons have greater safety concerns. Further, it
is suggested that women would simply feel more comfortable boarding an eVTOL with a
pilot [
64
] or at least some sort of security monitoring in the aircraft cabin, respectively [
60
].
4.4.2. Perceived Affordability
Regarding the perceived affordability, studies aim to forecast not the actual cost
of using a service for the individual (see chapter 4.1), but the threshold above which
average customers become unsatisfied with the pricing scheme and unwilling to pay for the
transport mode (cf. [
65
]). In alliance with prevailing transport planning approaches, this
willingness to pay is conceptualised as a customer’s trade-off between the value of travel
time savings and financial cost [
66
]. Building on this presumption, Balac et al. included
the option of Air Taxis in a mode choice simulation with agents representing 10% of the
population from the canton of Zurich, Switzerland. They researched the impact of varying
passenger handling times as well as travel speeds and costs. According to their research
on the sample, the willingness to pay decreases significantly above a base cost of CHF 6
and a cost per kilometre above CHF 1.8 [
67
] (equalising to around EUR 1.8 in Q1 2022).
For the USA, Goyal et al. modelled the sensitivity of customer demand to changes in
the flight price for 10,000 randomly generated air taxi missions in ten metropolitan areas
each and found that highest revenue in trade-off to customers’ decreasing willingness to
pay would be achieved at USD 2.50–2.85 per mile [
38
] (equalising to around EUR 1.5 to
1.7 per kilometre in Q1 2022). Concluding this, the price level up to which a broad customer
satisfaction with pUAM services is suggested is more than 50% below prices to be expected
from current estimations (cf. [
38
,
39
]). In addition, while it is commendable in terms of
social sustainability that broad segments of the population were targeted in the respective
research for acceptable pUAM pricing, Ahmed et al. emphasise the circumstance that the
willingness to pay and therefore the satisfaction with service prices is highly dependent
on the individual characteristics. For example, persons with an annual household income
over USD 100,000 are expected to be more willing to pay up to USD 6.5 per mile for pUAM
services [64], closely reaching the realistic cost estimation made by Goyal et al. [38].
4.4.3. Perceived Service Reliability
As research shows, the perceived service reliability, e.g., on-time performance [
60
] and
low performance risks [68] are significant for the adoption of air taxis and the willingness
to use all-electric passenger planes respectively. The latter decreases the more respondents
are concerned about the risk of not being able to complete their journey satisfactorily, that
problems during the journey cause cognitive stress, and/or that money will be lost due to
any concomitant circumstances [
68
]. Thus, unforeseen operational constraints in connection
Sustainability 2022,14, 9312 10 of 17
with pUAM would likely impact the perception of the reliability of pUAM significantly and
consequently, passenger satisfaction with the service. As shown in the previous chapter,
unexpected disruptions in vertiport operation may result in such unwanted events and
long passengers waiting. Additionally, airspace restrictions may prevent flights at short
notice, e.g., due to bad weather or due to congestion from other urban air space users [
52
].
4.4.4. Perceived Easiness to Use
Last but not least, the perceived easiness to use pUAM receives consideration in
research to improve customer acceptance and satisfaction. In the logic of the adopted indi-
cator, highest customer satisfaction can be assumed when an easy booking and payment
process is in place, vertiports can be accessed comfortably, waiting times are appropriate
and overall service quality is high [
11
,
60
,
61
]. Regarding the booking process, the integration
of pUAM into the MaaS environment is anticipated in most related research, which will
allow booking and paying for the complete travel chain using a digital platform [
7
]. Travel
can thus be expected to be comfortable for digital natives. Research on acceptable wait
times for pUAM was not found. However, as scheduled pUAM operation is anticipated in
this research and time saving is suggested to be a primary decision factor for customers,
a threshold for acceptable wait time should be reached where it will become worthwhile
for customers to choose another mode of transport in a given booking. As outlined before,
these wait times will be impacted foremost by vertiport operations and airspace access [
69
].
Regarding service quality, Edwards and Price in their research for NASA on eVTOL Passen-
ger Acceptance highlight several issues that could strongly impact customer satisfaction.
To name a few, feelings of anxiety could arise from in-fight turbulence and gust responses;
vehicle noise may cause discomfort; or outside-visuals may cause intimidation. Emphasis-
ing the yet small body of research on these aspects, the authors request further engagement
in this field to ensure “that the passenger’s first ride is not also their last” [13] p. 3.
4.5. Impact of pUAM on the Quality of Public Spaces
Similar to the evaluation of customer satisfaction, hints of a changing perception
towards public spaces through pUAM can only be sustained through survey data and
statistical model approaches. Regarding survey data, the before mentioned EASA study an-
ticipates various impacts of vertiports as necessary ground infrastructure on the perceived
quality of public spaces by the population. Respondents that were asked to rank their most
relevant concerns related to close-by vertiports in their surrounding area rated noise (48%)
and safety concerns (41%) most often. Furthermore, concerns regarding visual pollution
(32%), increased inbound and outbound traffic (29%) and the occupation of spaces better
used for living or recreation (28%) ranked high in concerns as well [
55
] p. 88. Regarding
the impact of air traffic in the urban sky, in the before mentioned Sky Limits survey from
Germany, 43% of respondents thought that air taxis would make urban spaces less pleasant
to live in while just 22% of respondents were certain that passenger transport with air taxis
would have a positive effect on the quality of life in cities. Asked about a future in which
many people were to use air taxis, 61% of respondents rated it very or quite bad if air taxis
would block the currently unobstructed view of the sky [58] p. 8, 14.
By creating a sample with 800 respondents from the same survey, Mostofi et al. devel-
oped a structural equation model to explain how the attitude of ordinary citizen towards
air taxis is formed. They find that the expected impact of pUAM on the overall quality
of life in cities is a significant predictor for the attitude respondents have towards eVTOL
operation in public spaces. Further, they observe aesthetic dimensions such as the blocked
view to the urban sky, noise and induced stress due to traffic movements above one’s head
as negatively impacting respondents’ attitude. Derived from these findings, the authors
advise pre-emptively minimising aesthetic risks in the choice and placement of vertiport
infrastructure, in vehicle routing and in route frequentation [70].
To achieve this, however, urban planning practice must first embrace the lower urban
airspace as a new subject for sustainable mobility planning. In this context, Kellerman et al.
Sustainability 2022,14, 9312 11 of 17
review aspects of urban planning and city development covered in the contemporary liter-
ature and conclude that local planning authorities are considered unprepared to integrate
three-dimensional air traffic (infrastructure) into existing planning practice. More precisely,
two lines of research have not yet been integrated into a comprehensive discourse. On one
hand, requirements for an UTM/U-Space are supposed to allow for high traffic volumes
and operational safety of drones. On the other hand, the authors quote requirements for
city planning authorities on the municipal or regional level to ensure a fair sharing of
societal burden and individual benefits from drone related services. Cooperation will be
required between different stakeholder groups such as commercial vertiport and air taxi
operators, civil society, affected residents and customers. Proposed as a tool to facilitate
this reconciliation are participatory planning practices [
4
]. However, it remains an open
research question what issues participatory processes can mitigate effectively and how they
can be implemented procedurally [
71
]. Relevant use cases are seen in the development of
community guidelines for drones, to factor stakeholder interests in U-Space planning [
56
]
or to mitigate drone related noise in a citizen science approach [72].
In the case of European regions and municipal authorities, the awareness for upcoming
urban planning challenges and potential solutions is limited, as UAM developments in
Europe have been focused strongly on model cities and regions. Those, however, have
already advocated for a deciding role in the governance of local urban airspace, e.g., on the
type of UAM services allowed as well as on the extent and territorial boundaries of services,
including the decision on no-fly zones and the placement of take-off and landing sites [
73
].
While this legal authority on the regional or municipal level may facilitate greater adaption
to local needs, other authors suspect the economic feasibility of pUAM to decrease due to
extensive operational restrictions within and between cities and regions [74].
5. Discussion
Based on the results presented above, impacts of pUAM on the social sustainability
rating of urban transportation systems in Europe can be discussed.
Findings on
affordability
provide a clear prospect. With about EUR 70 for a 10-km
inner-city short trip (cf. [
39
]), most citizens will find pUAM too expensive for regular use.
The introduction of pUAM would negatively affect the affordability rating of a European
urban transport system. Nevertheless, authors argue positively for the development of a
mass market and, along with it, decreasing consumer prices over the coming decades [
54
].
The current prime aspiration are public subsidies in the provision of local and regional
UTM/U-Spaces as well as for urban take-off and landing sites for cargo and passenger
drones. In analogy to conventional aviation air traffic management and road infrastructure,
it is argued that such public engagement may foster a return on investments due to
increased business activity [
5
]. In addition, greater engineering and operating efficiencies
are forecasted over the next decades. This may include: lower costs for batteries and
aircraft through mass production; extended aircraft operation through improved battery
capacity and rapid charging; the substitution of on-board pilots through autonomous flight
capability and a decreasing demand for staff on the ground through the automation of
passenger handling processes [
38
]. In contrast to these long-term forecasts, the evolvement
of a pUAM mass market may become hampered by improvements in alternative mobility
services that occur parallel to the maturation of pUAM. Those might favour a shift to
more efficient and sustainable transportation offers, e.g., ground vehicle automation in
connection with relevant network expansion, efficiency improvements and price reductions
in urban and regional ground transport. What is more, sustainable urban development
must be quoted as countervailing trend, leading to increased functional diversity, shorter
travel distances and less urban sprawl [7].
Drawing back on the literature regarding the
inclusivity of pUAM for mobility-
impaired groups
, the likelihood and extent to which inclusivity requirements will become
considered by eVTOL manufactures and vertiport architects cannot be assessed. Statements,
for example on whether it will be possible to accommodate mobility aids such as prams
Sustainability 2022,14, 9312 12 of 17
or wheelchairs were not found. To avoid economic liabilities in early market entrance for
the special designs and operational adjustments, the possibility is given, however, that
pUAM services will not adhere to accessibility standards as provided to mobility-impaired
groups in public transport. The inclusivity rating of the respective transport system would
thus decrease. Nevertheless, companies aiming to integrate pUAM in urban settings
must perforce cooperate with local planning authorities. Presuming their obligation to
ensure social equality in transportation, the compliance of pUAM providers with minimum
inclusivity standards might be enforceable [
33
]. What is more, dual use synergies from
eVTOLs for civil medical services and military emergency response might support inclusive
design features of certain vehicles [75].
Concluding the sub-chapter on the prospective
access to pUAM services
, the con-
struction of small vertiports for a pUAM touch-and-go configuration in inner-city districts
receives favourable regulations. However, available space significantly limits vertiport
capacities. In respect to the SUMI, this low capacity for inner-city pUAM would not mean-
ingfully impact the accessibility rating of the respective area. While the possibility should
not be excluded, the likelihood to which larger vertiports with more relevant throughput
rate will be placed central into attractive catchment areas seems far lower. Potential finan-
cial, legal and acceptance restraints connected to the construction and operation of larger
transport infrastructure in densely populated areas must be mitigated [
76
]. Adding the
envisioned integration of pUAM into the MaaS environment, it appears more likely that
feeder traffic such as taxi services must be used by customers to access and leave vertiports,
placed in less populated neighbourhoods. For vertiports in suburbs, outskirts and rural
areas the situation might prove different. Access to the transportation system may be
limited, favourable space for new infrastructures might be available and fewer legal and
social restrictions may apply, but so is customer demand for pUAM. A regular connection
of these stations as a prerequisite for improving the accessibility of the overall transport
system of a FUA would prove to be costly and unlikely be executed in demand-driven
pUAM planning approaches. To mitigate this shortcoming on frequency, authors point out
that vertiport operators may become open accessible transportation hubs, e.g., including
drone delivery and ground mobility services [
54
]. Especially for vertiports that are facing
less demand, such a model could lead to higher utilisation, more overall pUAM network
connections and thus increased accessibility for citizen.
Concluding the review on the prospective
satisfaction
of citizen with pUAM, safety
aspects must be considered especially significant for early market operation. Thereby,
studies suggest that the safety perception is rather subjective, meaning that pUAM can
be perceived as safe from one person who might have a higher trust in new technologies
while another feels more insecure and less willing to use the service [
60
]. The same
rationale is true for the perceived affordability of services, influenced by varying income
levels [
64
]. Consequently, the satisfaction with future pUAM in these dimensions will differ
amongst a population. Drawing on the literature, a broader satisfaction with pUAM in
society could be achieved when aiming for a high safety perception, e.g., having a pilot on
board (cf. [64])
and a pricing policy of around EUR 2 per kilometre and passenger (cf. [
67
])
while stable operation (cf. [
68
]) and travel time saving is ensured (cf. [
55
]). Nevertheless,
early business models must be anticipated to follow demand-driven approaches. Based on
the reviewed studies, the target group usually contains a high share of younger, technophile
males with higher education and income, living and frequently commuting individually
in FUAs [
11
,
37
,
59
,
60
,
64
]. Consequently, satisfaction with the urban transportation system
would only improve for this subgroup, while the majority of citizen will not feel considered.
From the existing literature regarding the
impact of UAM on the quality of public
spaces
it appears relevant to anticipate a negative impact of eVTOLs and infrastructure on
citizens’ perception towards the quality of urban public spaces. This impact might initially
be smaller due to low traffic density. However, external traffic costs such as noise and
stress for residents in the vicinity of vertiports and along corresponding flight corridors
should be highlighted when operation increases. Aesthetic demands for a clear urban sky,
Sustainability 2022,14, 9312 13 of 17
e.g., in recreational areas are suggested to play a significant role in connection to a high
sojourn quality. Privacy and safety concerns might additionally influence how citizens
perceive public spaces in which numerus aircrafts are deployed [
70
]. If or to what extent
the quality of public spaces deteriorates may depend on the balancing between economic
interests for a permissive airspace usage on the one hand and citizen-focused governance
on U-Space and vertiport planning on the other hand. From the point of analysis, it is seen
as favourable if municipal and regional planning authorities become key decision makers
regarding the configuration of their urban airspace to mitigate conflicting interests between
different stakeholders on the local level. Besides a corresponding legal framework, however,
urban planning competences would have to be strengthened and participatory procedures
developed (cf. [
4
]). In support of this, urban planning, sociology and human geographic
perspectives must be embraced in the sustainable development of urban airspace and its
ground infrastructure components [77].
6. Conclusions
The first step to steer passenger Urban Air Mobility (pUAM) towards the goal of
sustainable mobility is to better understand the impact of its introduction on our urban
transportation systems. This analysis focused on the prospective impact of pUAM on
the five dimensions of socially sustainable urban transportation as adopted from the
Sustainable Urban Mobility Indicators (SUMI) framework. Overall, it is to conclude that
the introduction of pUAM will have a rather negative impact on the social sustainability
assessment of European urban mobility systems. The short- to mid-term affordability of
pUAM for broad parts of the population cannot be expected without public subsidies.
For this engagement, however, local community must first demand clear prospects for
added value. Similarly, the overall inclusivity evaluation of urban transportation systems
must be expected to decline if planning authorities will not demand certain standards
for mobility-impaired groups. Vertiport operation in already developed urban locations
might not improve accessibility, however, cross-financed and open access mobility hubs in
suburbs and rural areas might include pUAM and thus contribute positively to the access
indicator. A high level of satisfaction with pUAM among the public is not expected due to
target-group specific business modelling. Last but not least, an impairment of the overall
quality of urban public spaces is likely but might be minimised through the allocation
of legal competences for urban airspace planning and civil society participation on the
local level.
Limitations: Due to the upsurge of literature on UAM and the selection of only two
databases, it may not have been possible to include all contributions in the field of research.
Nevertheless, a consistent picture of prospective planning requirements should have been
drafted. Further, while a share of the cited literature in this analysis emphasises futuristic
on-demand operation of autonomous air taxis, the frame of analysis in this research was
on early market, scheduled operation in European functionally urban areas. Thus, the
transferability of results may have been prone to errors. Finally, the results of this study
remain on a conceptual level. A future technology assessment in the presented categories
should be conducted as local case studies, factoring in the regional specifics, vehicle
characteristics and operational model.
Relevance and outlook: The expansion of metropolitan transportation to low-level
airspace seems pending. It is predictable that attractive European metropolitan areas
will be confronted with business concepts as soon as the legal framework will allow.
However, in analogy to the paradigm of sustainable urban development, the deployment
of passenger Urban Air Mobility should follow a holistic approach from the start. For
political stakeholders on the regional and local level, a forward-looking understanding on
the opportunities and risks associated with the new mobility offer will be key in making
confident decisions regarding an introduction. For planners, it becomes apparent that new
competencies and creative solutions will be required to steer urban air mobility towards
sustainable mobility for the common good. Chiefly, this challenge calls for the active
Sustainability 2022,14, 9312 14 of 17
engagement of civil society, as without stakeholder participation there is not only a risk of a
societal rejection and division, but also of decisions being made that do not realise positive
innovation potentials of this technology.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.
Acknowledgments:
I acknowledge support by the German Research Foundation and the Open
Access Publication Fund of TU Berlin.
Conflicts of Interest: The author declares no conflict of interest.
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