Discussion

In 2019, Americans took 9.9 billion trips on public transportation, which provides people with mobility and access to employment, community resources, medical care, and recreational opportunities in communities. ¹

Transport modalities, especially mass-transit systems, are vulnerable soft targets: easily accessible, relatively unprotected, and a place where a large number of civilians congregate. ^{Reference Shvetsov, Sharov and Kozyrev2} Attacks on such modalities can be highly lethal but can also inflict extensive psychosocial injuries on local populations, disrupt day-to-day community functions, and can lead to significant financial damage to both government and private organizations. ^{Reference Howitt and Makler3}

It is estimated that over 90% of public assistance recipients are dependent on public transportation and attacks on such systems, and the resulting disruptions to the service, will likely disproportionally affect the poor. ^{Reference Steinfeld, Maisel and Steinfeld4}

Population behavioral responses in the aftermath include generalized travel anxiety as well as avoiding public transport and shifting to individual modes of transport (which can result in increased congestion and higher rates of traffic-related incidents, trauma, and death). ^{Reference Elias, Albert and Shiftan5–Reference Ayton, Murray and Hampton7} Economic impacts include cleanup and repair costs but can also extend to travel and tourism activities.

While the airline industry has historically imposed more stringent security checks and regulations in regards to both domestic and international air travel, land-based mass-transport systems face the difficult dilemma of balancing the tradeoff between security considerations and service accessibility. ^{Reference Howitt and Makler3} Airline-style security screening and other target hardening measures would likely hinder the efficient flow of people but minimize and mitigate the risks of an attack.

While the weaponizing of transport modalities to commit terrorism acts are beyond the scope of this paper, events such as September 11, 2001 and other vehicular ramming and explosions such as the 2016 Nice, France truck attack have emerged as widely recognized terrorist methods, raising further concerns around the security of transport systems around the world. ^{Reference Bloom8}

The lethality of explosive attacks depend on several factors: density and distance of population in the vicinity, the duration and magnitude of the blast wave, open air versus confined space, open versus closed doors, diameter of tunnel (tube tunnel versus sub-surface tunnel), proximity to other vehicles or buildings, and potential tertiary injuries from collapsing structures; explosions near or within hard solid surfaces can be amplified multi-fold due to shock wave reflections and, as an example, collapse of the upper deck of a bus onto the lower deck will likely result in larger number of casualties. ^{Reference Turégano-Fuentes, Caba-Doussoux and Jover-Navalón9}

Furthermore, suicide bombing attacks can inflict a significantly higher death and injury toll than conventional attacks on any given transport modality. This is likely due to the agility of suicide bombers and their ability to move, change directions, infiltrate busy and populated areas relatively undetected, or change time and location of targets in real time. ^{Reference Tin, Galehan, Markovic and Ciottone10}

Subway attacks have been shown to be particularly lethal out of all the various transport modalities. Confined spaces with closed doors and limited evacuation paths are extremely prone to amplified pressure effects of blast attacks. While simple blast waves in open space create a rapid rise in air pressure, they tend to also rapidly dissipate, whereas confined space explosions with restricted venting can create complex patterns of shock wave reflections and interactions. Explosion-related high temperatures and inability for gaseous products to dissipate would produce more pressure, leading to prolonged blast exposures, higher pressures, and thus greater transfer of energy to the human body, increasing the risk of primary blast injuries such as blast lungs, belly, and brain as well as other organ trauma. Blast injuries are typically classified into Primary (blast wave), Secondary (debris displaced by blast wind), Tertiary (person displaced by blast wind or from surrounding structural collapse), and Quaternary injuries (any injuries that are not primary, secondary, or tertiary; eg, fire, smoke, and toxins). ^{Reference Adler and Rosenberger11}

Enclosed spaces with limited ventilation systems are also especially vulnerable to smoke inhalation injuries and chemical attacks. The 1995 Tokyo Subway Sarin gas attack remains one of the most significant terrorist acts in history, killing 12 and injuring over 5,500 others. ^{Reference Tin, Granholm, Hart and Ciottone12}

Emergency medical care responses to subway attacks are often hampered by communication failure and difficult accessibility, potentially delaying rescue and time to medical treatment. While advances in technology, communications, computational capabilities, and data platform integration have significantly improved disaster risk reduction strategies and help manage responses to mass-casualty incidents over the years, Tim O’Toole, former Managing Director of the London Underground (London, England), captured the importance of individuals in enacting the emergency response when he spoke to the Committee reviewing the 2005 London attacks: “the big lesson for us is to invest in your staff, rely on them; invest in technology, but do not rely on it.” ¹³

Although sophisticated counter-terrorism intelligence and technologies have evolved during recent years and attack risks can be mitigated through a number of relatively simple measures (such as installing metal detectors at entrances to public stations, security check points, and roadblocks), novel attack methodologies such as the use of drones need to be carefully considered and examined. ^{Reference Borrion, Tripathi, Chen and Moon14–Reference Tin, Hart and Ciottone16} Explosive-equipped drone swarms can be pre-programmed or manually flown into civilian air, land, and sea transport systems with little resistance and special considerations need to be taken when transporting dangerous or hazardous material goods. ^{Reference James17}

The “if you see something, say something” campaign was originally implemented in 2001 by the New York Metropolitan Transportation Authority (New York USA) to inspire, empower, and educate the public on suspicious activity reporting. By 2006, reports of suspicious packages in New York grew to over 37,000 and in 2010, the US Department of Homeland Security (Washington, DC USA) licensed the phrase for the purpose of creating a nation-wide campaign. The slogan has since been adopted by a multitude of national agencies across many cities around the world, including the Transportation Security Administration (Arlington, Virginia USA), Amtrak (Washington, DC USA), colleges, and sporting venues. ¹⁸