Table 1.

Emerging sub-themes detected by topic modeling analysis and indicative references.

Land Transport --- Operations

Sub-theme 1. Technological tools and railway operations

Keywords: Safety Technologies; Technological Solutions; Self-service Technology;

Network Capacity; High-speed Rail; Train Stations; Government Subsidy.

Ref.: Choi, 2020, Cserdi and Kenesei, 2020, Gaskin et al., 2020, Jiao et al., 2020;

Sub-theme 2. Control measures for urban traffic

Keywords: Travel Restrictions; Ridership Control; Pandemic Management; Contact Tracing;

Policy Recommendation; Transport Infrastructure; Transport Accessibility.

Ref.: Heo et al., 2020, Lee and Lee, 2020, Sharifi and Khavarian-Garmsir, 2020;

Allam and Jones, 2020, De Vos, 2020, Love et al., 2020, Mogaji, 2020;

Sub-theme 3. Non-motorized mobility

Keywords: Bike Sharing Systems; Demand-responsive Transport System; Digital Mobility;

Innovative Travel Modes; Mobile Crowdsensing; Safe Routes; Cycling.

Ref.: Barbarossa, 2020, Bruzzone et al., 2020, Campisi et al., 2020, Teixeira and Lopes, 2020;

Sub-theme 4. Public transit

Keywords: Public Transport Planning; Occupant Exposure; Indoor Exposure; Route Guidance;

Risk Management; Traffic Scenarios; Complex Networks.

Ref.: Ceder and Jiang, 2020, Mo et al., 2020, Romero et al., 2020, Ronchi and Lovreglio, 2020;

Land Transport — Traffic Demand

Sub-theme 1. Protective behavior and working from home

Keywords: Short-distance Trips; Self-restriction; Psychological Risk; Intentions to Use;

Health Protection Behavior; Telecommuting; Telework; Working From Home.

Ref.: Beck and Hensher, 2020, Shamshiripour et al., 2020, Wang and Ozbilen, 2020;

Dong et al., 2020, Hensher et al., 2020, Hotle et al., 2020, Parady et al., 2020;

Sub-theme 2. Spreading speed and traffic flow

Keywords: Spread Pattern; Disease Transmission; Traffic Volume; Mobile Phone Data;

Inter-city Travel; Population Flow; Congestion; Network Configuration.

Ref.: Arimura et al., 2020, Fang et al., 2020, Saladié et al., 2020, Shen, 2020;

Glaeser et al., 2020, Lawal and Nwegbu, 2020, Parr et al., 2020, Xi et al., 2020;

Sub-theme 3. Sustainability of urban mobility

Keywords: Sustainable Mobility; Sustainability Transitions; Ride Sharing; E-scooters;

Driving Behavior; Private Vehicles; Transportation expenses; Elderly.

Ref.: Button et al., 2020, Pantelaki et al., 2020, Schippl and Truffer, 2020;

Assoumou Ella, 2020, Bucsky, 2020, Cochran, 2020, Scorrano and Danielis, 2020;

Air Transport

Sub-theme 1. Aviation’s recovery flight plan

Keywords: Airport Service; Forecast of Air Flow; Market Recovery; Operational Efficiency;

Future Tourism Demand; Importation Risk; Exportation Risk; Complex Networks.

Ref.: Gallego and Font, 2020, Gudmundsson et al., 2021, Hotle and Mumbower, 2021;

Czerny et al., 2021, Iacus et al., 2020, Nakamura and Managi, 2020, Zhang et al., 2020;

Sub-theme 2. Passenger preferences and willingness to fly

Keywords: Outbreak Dynamics; Airline Protocol; Passenger’s Behavioral Changes;

Willingness to Fly; Risk Assessment; Passenger Health Filter.

Ref.: Graham et al., 2020, Gunay and Gokasar, 2021, Pongpirul et al., 2020;

Lamb et al., 2020, Oztig and Askin, 2020, Tabares, 2021;

Sub-theme 3. Safety and security in aviation

Keywords: Airline Strategy; Service Quality; Airplane Boarding; Airplane Seat Assignment;

Carrier’s Liability; Security Control; Passenger Flow Simulation.

Ref.: Dabachine et al., 2020, Milne et al., 2020a, Milne et al., 2020b, Salari et al., 2020;

Di Mascio et al., 2020, Kierzkowski and Kisiel, 2020, Naboush and Alnimer, 2020;

Sub-theme 4. Airlines’ strategic responses

Keywords: Cash Flow; Liquidity; Exogenous Shocks; Event Study; Non-market strategy;

Government Support; Regulated/Unregulated Airports.

Ref.: Abate et al., 2020, Akbar and Kisilowski, 2020, Albers and Rundshagen, 2020;

Budd et al., 2020, Gössling, 2020, Sobieralski, 2020, Suau-Sanchez et al., 2020;

Environment

Sub-theme 1. Changes in concentration of air pollutants

Keywords: Vehicle Fleet; Traffic emissions; Fine Particulate Matter; Air Quality Index;

Pollution reduction; Nitrogen Dioxide (NO${}_2$); Ozone (O${}_3$); Monitoring Stations.

Ref.: Latif et al., 2020, Lian et al., 2020, Şahin et al., 2020, Wang et al., 2020;

Kumar et al., 2020, Menut et al., 2020, Singh et al., 2020, Zhao et al., 2020;

Sub-theme 2. Air transport mobility and noise emissions

Keywords: Urban Noise Pollution; Noise Mapping; Sound Reduction;

Airport Emissions; Black Carbon; Air traffic.

Ref.: Aletta et al., 2020, Basu et al., 2020, Nižetić, 2020, Sobieralski and Hubbard, 2020;

Sub-theme 3. Energy systems and renewable fuels

Keywords: Energy System Balancing; Electric Vehicles; Hybrid-Engine Vehicles;

Fuel Consumption; Energy demand; Gasoline Passenger Cars.

Ref.: Chiaramonti and Maniatis, 2020, Sui et al., 2020, Tian et al., 2020, Xian-Chun et al., 2020;