	Introduction

The nation's air, land, and marine transportation systems are designed for accessibility and efficiency, two characteristics that make them vulnerable to attack. While hardening the transportation sector against terrorist attack is difficult, measures can be taken to deter terrorists. The focus of debate is how best to construct and finance a system of deterrence, protection, and response that effectively reduces the possibility and consequences of terrorist attacks without unduly interfering with travel, commerce, and civil liberties. Aviation Security1

Following the 9/11 terrorist attacks, Congress took swift action to create the Transportation Security Administration (TSA), federalizing all airline passenger and baggage screening functions and deploying significantly increased numbers of armed air marshals on commercial passenger flights. Despite the extensive focus on aviation security for more than a decade, a number of challenges remain, including
effectively screening passengers, baggage, and cargo for explosives threats; developing effective risk-based methods for screening passengers and others with access to aircraft and sensitive areas; exploiting available intelligence information and watchlists to identify individuals who pose potential threats to civil aviation; effectively responding to security threats at airports and screening checkpoints; developing effective strategies for addressing aircraft vulnerabilities to shoulder-fired missiles and other standoff weapons; and addressing the potential security implications of unmanned aircraft operations in domestic airspace. In addition to continued deployment and utilization of AIT, the FAA Extension, Safety, and Security Act of 2016 ( P.L. Risk-Based Passenger Screening

TSA has initiated a number of risk-based screening initiatives to focus its resources and apply directed measures based on intelligence-driven assessments of security risk. One concern raised over the PreCheck program is the lack of biometric authentication to verify participants at screening checkpoints. In July 2012, TSA expanded the program to include flight attendants. TSA requires security background checks of airport workers with unescorted access privileges to secure areas at all commercial passenger airports and air cargo facilities. The use of explosives detection technology was mandated by the Aviation and Transportation Security Act (ATSA; P.L. Transit and Passenger Rail Security43

Bombings of and shootings on passenger trains in Europe and Asia have illustrated the vulnerability of passenger rail systems to terrorist attacks. Passenger rail systems—primarily subway systems—in the United States carry about five times as many passengers each day as do airlines, over many thousands of miles of track, serving stations that are designed primarily for easy access. In an October 2015 hearing, DHS officials reiterated their opposition to a 100% scanning strategy in favor of a risk-based and layered security strategy. Transportation Worker Identification Credential (TWIC)

In January 2007, TSA and the Coast Guard issued a final rule implementing the Transportation Worker Identification Credential (TWIC) at U.S. ports.
The nation's air, land, and marine transportation systems are designed for accessibility and efficiency, two characteristics that make them highly vulnerable to terrorist attack. While hardening the transportation sector from terrorist attack is difficult, measures can be taken to deter terrorists. The dilemma facing Congress is how best to construct and finance a system of deterrence, protection, and response that effectively reduces the possibility and consequences of another terrorist attack without unduly interfering with travel, commerce, and civil liberties.
Aviation security has been a major focus of transportation security policy since the terrorist attacks of September 11, 2001. In the aftermath of these attacks, the 107th Congress moved quickly to pass the Aviation and Transportation Security Act (ATSA; P.L. 107-71), creating the Transportation Security Administration (TSA) and mandating a federalized workforce of security screeners to inspect airline passengers and their baggage. The FAA Extension, Safety, and Security Act of 2016 (P.L. 114-190) included a number of aviation security provisions designed to expand the PreCheck program to expedite screening for known travelers, enhance background checks of airport workers and strengthen airport access controls, and improve passenger checkpoint efficiency and operational performance.
Until recently, TSA applied relatively uniform methods to screen airline passengers, focusing primarily on advances in screening technology to improve security and efficiency. TSA has recently shifted away from this approach, which assumes a uniform level of risk among all airline travelers, to risk-based screening approaches that focus more intensely on passengers thought to pose elevated security risks. Despite the extensive focus on aviation security over the past decade, a number of challenges remain, including
effectively screening passengers, baggage, and cargo for explosives threats; developing effective risk-based methods for screening passengers and airport workers with access to aircraft and sensitive areas; exploiting available intelligence information and watchlists to identify individuals who pose potential threats to civil aviation; effectively responding to security threats at airports and screening checkpoints; developing effective strategies for addressing aircraft vulnerabilities to shoulder-fired missiles and other standoff weapons; and addressing the potential security implications of unmanned aircraft operations.
Bombings of passenger trains in Europe and Asia in the past few years illustrate the vulnerability of passenger rail systems to terrorist attacks. Passenger rail systems—primarily subway systems—in the United States carry about five times as many passengers each day as do airlines, over many thousands of miles of track, serving stations that are designed primarily for easy access. Transit security issues of recent interest to Congress include the quality of TSA's surface transportation inspector program and the slow rate at which transit and rail security grants have been expended.
Existing law mandates the scanning of all U.S.-bound maritime containers with non-intrusive inspection equipment at overseas ports of loading by July 2012. This deadline was not met, and DHS is opposed to that strategy in favor of a risk-based, layered approach to security screening. Implementation of the Transportation Worker Identification Credential (TWIC) for port and maritime workers also appears to be experiencing continuing difficulties.