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Regional ITS Architecture Guidance Document

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Appendix C: Glossary


A framework within which a system can be built. Requirements dictate what functionality the architecture must satisfy. An Architecture functionally defines what the pieces of the system are and the information that is exchanged between them. An Architecture is functionally oriented and not technology specific which allows the Architecture to remain effective over time. It defines "what must be done," not "how it will be done".

Architecture Baseline

The clear identification of the architecture products that will be maintained, including specific format and version information. Changes to the Architecture Baseline must follow an approved change management process typically documented in a Maintenance Plan. The Architecture Baseline will change over time as the Architecture is revised.

Architecture Flow

Information that is exchanged between Subsystems and Terminators in the Physical Architecture view of the National ITS Architecture. Architecture Flows are the primary tool that is used to define the Regional ITS Architecture Interfaces. These Architecture Flows and their communication requirements define the Interfaces, which form the basis for much of the ongoing Standards work in the National ITS program. In this document, the terms "information flow" and "architecture flow" are used interchangeably.

Architecture Interconnect

Communications paths that carry information between Subsystems and Terminators in the Physical Architecture view of the National ITS Architecture. Several different types of Interconnects are defined in the National ITS Architecture to reflect the range of Interface requirements in ITS. The majority of the interconnects are various types of communications links that are defined in the communications layer. Four different types of communications links are defined: Fixed-Point to Fixed-Point, Wide Area Wireless, Dedicated Short Range Communications, and Vehicle to Vehicle communications. In addition to these types, several specialized Interconnects are also defined to reflect other Interface requirements, including human interface (e.g., what the system user sees and hears) and physical/environmental (e.g., what the ITS sensors sense).

Center Subsystems

Subsystems that provide management, administrative, and support functions for the transportation system. The Center Subsystems each communicate with other centers to enable coordination between modes and across jurisdictions. Some examples of center subsystems are Traffic Management, Transit Management, Commercial Vehicle Administration, Planning, Emissions Management, Toll Administration, Emergency Management, Information Service Provider, and Fleet and Freight Management. One of four general Subsystem classes defined in the National ITS Architecture.


Person or persons who serve as a point-of-contact and provide leadership in the development, use, and maintenance of a Regional ITS Architecture.

Communications Hub

A communications hub facilitates communications connectivity between regional ITS architecture systems. Hubs normally provide communications and server functions and may or may not provide transportation-related functions such as data fusion.

Congestion Management Plan

Document that describes the strategy that will be used to prevent deterioration of Congestion Management System (CMS) performance.

Congestion Management System (CMS)

TEA-21 required that each Transportation Management Area (see definition of TMA) develop a CMS that provides for effective management of new and existing transportation facilities through the use of travel demand reduction and operational management strategies. This was replaced by a Congestion Management Process in SAFETEA-LU.

Corridor/Sub-area Study

Also known as "Major Investment Studies," these studies are used to flesh out transportation strategy and project recommendations on a geographic basis. A Corridor or Sub-area is a context for evaluating how specific transportation conditions, problems, and needs should be addressed within the defined geographic area. A wide range of multimodal strategies, including ITS, are considered as candidate solutions for those problems.

Data Flow

Data flows represent a pipeline along which information of known composition is passed. Data Flows are modeled in the Logical Architecture view of the National ITS Architecture. Data Flows represent data flowing between Processes or between a Process and a terminator. A Data Flow is shown as an arrow on a Data Flow Diagram and is defined in a Data Dictionary Entry in the Logical Architecture, including a textual description of the Data Flow and any lower level data elements that make up the Data Flow. Data flows are aggregated together to form high-level Architecture Flows in the Physical Architecture view of the National ITS Architecture.

Data Flow Diagram

The diagrams in the Logical Architecture view of the National ITS Architecture that show the functions that are required for ITS and the data that moves between these functions. Only four different symbols are used on the diagrams. Circles represent the Processes or functions that do the work. Arrows represent the Data Flows that show how data moves through the system. Parallel lines represent Data Stores that represent "data at rest" in the system. Finally, rectangles represent the terminators that define the Architecture boundary. A hierarchy of these diagrams depict the ITS functionality and Data Flow requirements in successively greater detail.

Dedicated Short Range Communications (DSRC)

A wireless communications channel used for close-proximity communications between vehicles and the immediate infrastructure. It supports location-specific communications for ITS services such as toll collection, transit vehicle management, driver information, and automated commercial vehicle operations. One of the types of Architecture Interconnects defined in the National ITS Architecture.


This is the basic building block of Regional ITS Architectures and Project ITS Architectures. It is the name used by the Stakeholders to describe a system or piece of a system.

Equipment Package

Equipment Packages are the building blocks of the Physical Architecture Subsystems. Equipment Packages group like Processes of a particular Subsystem together into an "implementable" package. The grouping also takes into account the User Services and the need to accommodate various levels of functionality. Since Equipment Packages are both the most detailed elements of the Physical Architecture view of the National ITS Architecture and tied to specific Market Packages, they provide the common link between the interface-oriented Architecture definition and the deployment-oriented Market Packages.

Federal Highway Administration (FHWA)

An agency of the United States Department of Transportation that funds highway planning and programs.

Federal Transit Administration (FTA)

An agency of the United States Department of Transportation that funds transit planning and programs.

Field Subsystems

One of four general classes of Subsystems defined in the National ITS Architecture. This class includes the intelligent infrastructure distributed along the transportation network which perform surveillance, information provision, and control functions. This includes ITS field equipment on roadway facilities as well as ITS equipment at parking facilities, toll systems, security monitoring systems, and commercial vehicle check systems that are also at or near the roadside.

Fixed-Point to Fixed-Point Communications

A communications link serving stationary entities. It may be implemented using a variety of public or private communications networks and technologies. It can include, but is not limited to, twisted pair, coaxial cable, fiber optic, microwave relay networks, spread spectrum, etc. In Fixed-Point to Fixed-Point (FP2FP) communication the important issue is that it serves stationary entities. Both dedicated and shared communication resources may be used. One of the four types of Architecture Interconnects defined in the National ITS Architecture.

Functional Requirements

A description of WHAT a system must do to address the needs or provide the services that have been identified for the region. The description should use formal "shall" language and document the functions in terms that the stakeholders, particularly the system implementers, will understand. In a Regional ITS Architecture, the Functional Requirements focus on the high-level requirements that support regional integration. For a project ITS architecture, these are broken down into more detailed requirements to fully understand the functionality of the system.

Highway Trust Fund (HTF)

Established as a federal mechanism for financing the accelerated highway program, the HTF is a source of funds from which ISTEA, TEA-21, and now SAFETEA-LU authorizes expenditures.


See Communications Hub.

Information Flow

Information that is exchanged between Subsystems and Terminators in the Physical Architecture view of the National ITS Architecture. In this document, the terms "Information Flow" and "Architecture Flow" are used interchangeably.

Institutional Integration

Institutional Integration represents the process of combining existing and emerging institutional constraints and arrangements. Integration is at least two-fold in a region; technical integration involves the functional act of integration while institutional integration addresses the agency and/or regional environment for integration. Both are necessary components for interoperable systems.

Intelligent Transportation System (ITS)

Electronics, communications, and information processing used singly or integrated to improve the efficiency or safety of surface transportation.

ITS Architecture

Defines an Architecture of interrelated systems that work together to deliver transportation Services. An ITS Architecture defines how systems functionally operate and the interconnection of information exchanges that must take place between these systems to accomplish transportation Services.

ITS Project

Any project that in whole or in part funds the acquisition of technologies or systems of technologies that provide or significantly contribute to the provision of one or more ITS User Services.

ITS Strategic Plan

A guide for long term implementation of ITS in the state, metropolitan area, or region. A Strategic Plan will normally include identifying regional transportation needs and then defining ITS Elements to be implemented over time, aimed at meeting those needs. A regional ITS architecture is typically a core component of an ITS Strategic Plan.


See Architecture Interconnect.


The connection between two systems. In the regional ITS architecture, an interface is described by the architecture interconnect – the line of communications between the two systems – and the information flows that define the types of information that will be shared over the interconnect.

Intermodal Surface Transportation Efficiency Act of 1991 (ISTEA)

Legislative initiative by the United States Congress that restructured funding for transportation programs. ISTEA authorized increased levels of highway and transportation funding and an increased role for regional planning commissions/MPOs in funding decisions. The Act also requires comprehensive Regional and Statewide long-term Transportation Plans and places an increased emphasis on public participation and transportation alternatives.


See System Inventory.

Joint Program Office (JPO)

The office of the United States Department of Transportation established to oversee and guide the multi-modal National Intelligent Transportation Systems program.

Legacy System

Existing transportation systems, communications systems, or institutional processes.


A term used when denoting a progression through a series or sequence of differing stages of development.

Logical Architecture

The Logical Architecture view of the National ITS Architecture defines what has to be done to support the ITS User Services. It defines the Processes that perform ITS functions and the information or Data Flows that are shared between these Processes. The Logical Architecture was developed using Structured Analysis techniques and consists of Data Flow Diagrams, Process Specifications, and Data Dictionary Entries. The Logical Architecture is not technology specific, nor does it dictate a particular implementation. This implementation independence makes the Logical Architecture accommodating to innovation, scaleable from small-scale implementations to large regional systems, and supportive of widely varied system designs.

Maintenance Plan

Description of configuration control and update guidelines for Regional and/or Project ITS Architectures. The primary purpose of the Maintenance Plan is to maintain an Architecture Baseline.

Major ITS Project

Any ITS project that implements part of a regional ITS initiative that is multi-jurisdictional, multi-modal, or otherwise affects regional integration of ITS systems.

Market Package

Market Packages identify the pieces of the Physical Architecture that are required to implement a particular transportation Service. They provide an accessible, service oriented, perspective to the National ITS Architecture. They are tailored to fit – separately or in combination – real world transportation problems and needs. Market Packages collect together one or more Equipment Packages that must work together to deliver a given transportation Service and the Architecture Flows that connect them and other important external systems.

Memorandum of Understanding (MOU)

Institutional agreement that contains a description of the level of effort and integration details necessary for sharing information between ITS elements. An MOU can also address sharing of equipment, maintenance, enforcement, operations, control, etc. An MOU can be used to create a more detailed agreement such as an Interagency, Intergovernmental, Operational, Funding or Master Agreement.

Metropolitan Planning Organization (MPO)

The forum for cooperative decision making for the metropolitan planning area.

Metropolitan Transportation Plan

This document is the official metropolitan, intermodal transportation plan that is developed through the regional transportation process.

National ITS Architecture

A common, established framework for developing integrated transportation systems. The National ITS Architecture is comprised of the Logical Architecture and Physical Architecture, which satisfy a defined set of User Services. The National ITS Architecture is maintained by the United States Department of Transportation (USDOT).

National Program Plan

Jointly developed by the USDOT and ITS America with substantial involvement from the broader ITS community. The purpose of the National Program Plan was to guide the development and deployment of ITS. It defined the first 29 User Services that were the basis for the National ITS Architecture development effort.

Operational Concept

An operational concept identifies the roles and responsibilities of participating agencies and stakeholders. It defines the institutional and technical vision for the region and describes how the system will work at a very high-level, frequently using operational scenarios as a basis.

Physical Architecture

The Physical Architecture is the part of the National ITS Architecture that provides agencies with a physical representation (though not a detailed design) of the important ITS Interfaces and major system components. The principal elements in the Physical Architecture are the Subsystems and Architecture Flows that connect these Subsystems and Terminators into an overall framework. At the next level of detail, the Physical Architecture identifies Equipment Packages for each Subsystem and assigns Processes identified in the Logical Architecture to these Equipment Packages. Similarly, the Data Flows from the Logical Architecture are grouped together and assigned to Architecture Flows.

Physical Entities

Persons, places, and things that make up an intelligent transportation system. In the physical architecture, an entity represents a National ITS Architecture subsystem or terminator.


A function or activity identified in the Logical Architecture view of the National ITS Architecture that is required to support the ITS User Services. The Logical Architecture presents Processes in a top-down fashion beginning with general Processes (e.g., "Manage Traffic") that are then decomposed into more detailed Processes (e.g., "Provide Traffic Surveillance", "Monitor HOV Lane Use"). General Processes are defined in terms of more detailed Processes using Data Flow Diagrams. The most detailed Processes are defined in Process Specifications (Pspecs).

Process Specification (Pspec)

The textual definition of the most detailed Processes identified in the Logical Architecture view of the National ITS Architecture. The Process Specification includes an overview, a set of functional requirements, a complete set of inputs and outputs, and a list of the User Service requirements that are satisfied by the Pspec.

Project ITS Architecture

A framework that identifies the institutional agreement and technical integration necessary to define an ITS project and its interfaces with other ITS projects and systems.

Project Sequencing

The order in which projects are deployed. An important part of the transportation planning process is the sequence or order that ITS projects are deployed. The Regional ITS Architecture provides a new way to look at these ITS projects relationships or "dependencies". By taking these dependencies into account, an efficient sequence can be developed so that projects incrementally build on each other.


Abbreviation for Process Specification.


The geographical area that identifies the boundaries of the Regional ITS Architecture and is defined by and based on the needs of the participating agencies and other Stakeholders. In metropolitan areas, a Region should be no less than the boundaries of the metropolitan planning area.

Regional ITS Architecture

A specific, tailored framework for ensuring institutional agreement and technical integration for the implementation of ITS projects or groups of projects in a particular Region. It functionally defines what pieces of the system are linked to others and what information is exchanged between them.

Safe, Accountable, Flexible, Efficient Transportation Equity Act – A Legacy for Users (SAFETEA-LU)

Passed in 2005, this act reauthorizes federal surface transportation programs through the end of FY2009. SAFETEA-LU is the successor to TEA-21.

Service Boundaries

The geographic boundary of a specific service or agency that provides a service. An example is the service area of a transit agency. The transit agency provides services only within the defined boundary.


Anyone with a vested interest or "stake" in one or more transportation elements within a regional ITS architecture. This includes public agencies, private organizations, special interests, and the traveling public.


Documented technical specifications sponsored by a Standards Development Organization (SDO) to be used consistently as rules, guidelines, or definitions of characteristics for the interchange of data. A broad array of ITS Standards is currently under development that will specifically define the Interfaces identified in the National ITS Architecture.

State Implementation Plan (SIP)

A document required to be prepared periodically by each state that contain one or more "air-quality non-attainment" areas that describes specific projects that will be constructed and/or operated over the next several years.

Statewide Transportation Improvement Program (STIP)

This is a document prepared by each state that is a staged, multi-year, statewide, intermodal program of transportation projects which is consistent with the Statewide Transportation Plan and planning processes and Metropolitan Transportation Plans, TIPs and processes.

Statewide Transportation Plan

This document is the official statewide, intermodal transportation plan that is developed through the statewide transportation process.


The principal structural element of the Physical Architecture view of the National ITS Architecture. Subsystems are individual pieces of the Intelligent Transportation System defined by the National ITS Architecture. Subsystems are grouped into four classes: Centers, Field, Vehicles, and Travelers. Example Subsystems are the Traffic Management Subsystem, the Vehicle Subsystem, and the Roadway Subsystem. These correspond to the physical world: respectively traffic operations centers, automobiles, and roadside signal controllers. Due to this close correspondence between the physical world and the Subsystems, the Subsystem interfaces are prime candidates for standardization.

Subsystem Diagram

A diagram which depicts all Subsystems in the National ITS Architecture and the basic communication channels between these Subsystems. This diagram, sometimes informally referred to as a "Sausage" diagram, is a top-level Architecture Interconnect diagram. Variations of this diagram are sometimes used to depict Regional ITS Architectures at a high level.


A collection of hardware, software, data, processes, and people that work together to achieve a common goal. Note the scope of a "system" depends on one's viewpoint. To a sign manufacturer, a dynamic message sign is a "system". To a state DOT, the same sign is only a component of a larger Freeway Management "System". In a Regional ITS Architecture, a Freeway Management System is a part of the overall surface transportation "system" for the region.

System Inventory

The collection of all ITS-related Elements in a Regional ITS Architecture.

Systems Engineering

An interdisciplinary approach and means to enable the realization of successful systems. Systems engineering is a systematic approach that focuses on defining customer needs and required functionality early in the development cycle, documenting requirements, then proceeding with design synthesis and system validation while considering the complete problem.


Terminators define the boundary of an Architecture. The National ITS Architecture Terminators represent the people, systems, and general environment that interface to ITS. The Interfaces between Terminators and the Subsystems and Processes within the National ITS Architecture are defined, but no functional requirements are allocated to Terminators. The Logical and Physical Architecture views of the National ITS Architecture both have exactly the same set of Terminators.

Transit Development Plan

This document is prepared by the transit agency and identifies transit needs and the changes in services and systems that are recommended to better meet those needs. It will feed information to both the Transportation Plan and the Transportation Improvement Program (TIP).

Transportation Equity Act for the 21st Century (TEA-21)

Passed in 1997 by Congress to address the need to begin work toward regional integration of transportation systems. TEA-21 was the successor reauthorization of the ISTEA legislation and has now been succeeded by SAFETEA-LU.

Transportation Improvement Program (TIP)

This is a document prepared by each Metropolitan Planning Organization (MPO) listing projects to be funded with FHWA/FTA funds for the next one to three year period. It is consistent with the Metropolitan Transportation Plan.

Transportation Management Area (TMA)

All urbanized areas over 200,000 in population and all other areas that request designation.

Transportation Plan

Also called the "Long Range Transportation Plan", this plan defines the state or metropolitan area's long-term approach to constructing, operating, and maintaining the multi-modal transportation system.

Traveler Subsystems

Equipment used by travelers to access ITS services pre-trip and en-route. This includes services that are owned and operated by the traveler as well as services that are owned by transportation and information providers. One of four general Subsystem classes defined in the National ITS Architecture.

Turbo Architecture

An automated software tool used to input and manage System Inventory, Market Packages, Interconnects and Architecture Flows with regard to a Regional ITS Architecture.

United States Department of Transportation (USDOT)

The principal direct Federal funding agency for transportation facilities and programs. Includes the Federal Highway Administration (FHWA), the Federal Transit Administration (FTA), the Federal Railroad Administration (FRA), the Federal Motor Carrier Safety Administration (FMCSA), and others.

User Service

User Services document what ITS should do from the user's perspective. A broad range of users are considered, including the traveling public as well as many different types of system operators. User Services, including the corresponding user service requirements, form the basis for the National ITS Architecture development effort. The initial User Services were jointly defined by USDOT and ITS America with significant Stakeholder input and documented in the National Program Plan (NPP). The concept of user services allows system or project definition to begin by establishing the high level services that will be provided to address identified problems and needs. Over time, new or updated User Services will continue to be developed and the National ITS Architecture will be updated to support these User Service changes.

User Service Requirement

A specific functional requirement statement of what must be done to support the ITS User Services. The User Service Requirements were developed specifically to serve as a requirements baseline to drive National ITS Architecture development. The User Service Requirements are not requirements to system/architecture implementers, but rather are directions to the National ITS Architecture development team.

Vehicle Subsystems

Covers ITS related elements on vehicle platforms. Vehicle Subsystems include general driver information and safety systems applicable to all vehicle types. Three fleet Vehicle Subsystems (Transit, Emergency, and Commercial Vehicles) add ITS capabilities unique to these special vehicle types. One of four general Subsystem classes defined in the National ITS Architecture.

Vehicle-to-Vehicle Communications

Dedicated wireless system handling high data rate, low probability of error, line of sight communications between vehicles. Advanced vehicle services may use this link in the future to support advanced collision avoidance implementations, road condition information sharing, and active coordination to advanced control systems. One of the four types of Architecture Interconnects defined in the National ITS Architecture.

Wide Area Wireless Communications

A communications link that provides communications via a wireless device between user and an infrastructure based system. Both broadcast (one-way) and interactive (two-way) communications services are grouped into wide-area wireless communications in the National ITS Architecture. These links support a range of services in the National ITS Architecture including real-time traveler information and various forms of fleet communications. One of the four types of Architecture Interconnects defined in the National ITS Architecture.

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