The Transportation Layer defines the functions that are performed, the subsystems that provide these functions, and the interfaces that are required to support the ITS User Services. This layer, the heart of the National ITS Architecture, provides a framework for applying technology in a consistent, progressive, and effective fashion to improve the surface transportation system. The majority of the National ITS Architecture definition focuses on the Transportation Layer. The Physical Architecture defines the subsystems and interfaces, the Logical Architecture defines the functions that are performed and the data that is exchanged between functions, and the Service Packages provides a menu of the transportation services that are provided.
In addition to subsystems, the Transportation Layer also defines terminators that represent all of the other systems, people, and physical conditions that the surface transportation system must interface with. The terminators describe the architecture context, that is, what functions are defined in the architecture and what is considered outside the scope of the architecture. The functions defined in the architecture are contained within subsystems. The functions at the boundary of the architecture are represented by terminators. These two types of architectural components (subsystems and terminators) are called entities.
The entities are grouped into four classes:
- Center — Centers provide management, administration, and support functions for the transportation system. The centers each communicate with other centers to enable coordination between modes and across jurisdictions within a region. The centers also communicate with field and vehicle classes to gather information and provide information and control that is coordinated by the centers.
- Field — The entities in this class provide the direct interface to the roadway network, vehicles traveling on the roadway network, and travelers in transit. They support direct surveillance, information provision, and control plan execution in the surface transportation system. All field subsystems interface to one or more of the center subsystems that govern overall operation of the field subsystems. The field subsystems also generally include direct user interfaces to drivers and transit users and short-range interfaces to the Vehicle Subsystems to support operations.
- Vehicle — These entities that are all vehicle-based and share many general driver information, vehicle navigation, and advanced safety systems functions. The vehicle subsystems communicate with the field subsystems and center subsystems for provision of information to the driver.
- Traveler — These entities include the equipment that is typically owned and operated by the traveler. Though this equipment is often general purpose in nature and used for a variety of tasks, this equipment is specifically used for gaining access to traveler information within the scope of the ITS architecture. These subsystems interface to the information provider (one of the center subsystems, most commonly the Information Service Provider Subsystem) to access the traveler information. A range of service options and levels of equipment sophistication are considered and supported. Specific equipment included in this class include personal computers, smart phones, tablets, and any other communications-capable consumer products that can be used to supply information to the traveler.
Another way of categorizing entities is by the entity type. There are four types of physical entities defined in the Transportation Layer:
- System — A computer system that accepts inputs, produces outputs, and performs some function, whether transportation-related or not. For example, a Traffic Management Subsystem manages traffic, and financial transactions are handled by a Financial Institution terminator.
- People — People who interact with the surface transportation system. These people could either be travelers who use ITS to achieve travel goals, or operators of ITS who use features to streamline their operations, improve service, or make money. Each interface to a user involves human interaction with the system.
- Other System — there may be multiple instances of each of the architecture subsystems. This type is used to show the interaction between these multiple instances; for example, the interface between two Traffic Management Subsystems.
- Environment — the physical world of pavement, weather conditions, obstacles and so on.
Interfaces between the entities are most commonly data exchanges that can be carried by communication media, but some interfaces are fuzzier, representing physical observation, contact, or human interaction. The interfaces often represent not only physical connections but also institutional interfaces between operating agencies. Careful definition of these interfaces provides developers with an understanding of how to build components that will reliably integrate with other components in future ITS deployments. In defining the Transportation Layer, the architecture development team has been careful to not over specify an ITS design. It is impossible to foresee what technology will be forthcoming or what roles agencies wish to play in ITS in each region. Therefore, the architecture remains flexible indicating top level data that are exchanged and basic functions that are performed. It leaves the specific system design up to implementers and the interface standards development up to standards development bodies and stakeholders with specific domain knowledge and vested interest in the outcome.