Access Control

Refers to mechanisms and policies that restrict access to computer resources. An access control list (ACL), for example, specifies what operations different users can perform on specific files and directories.

Access Management

The control and regulation of the spacing and design of driveways, medians, median openings, traffic signals and intersections on arterial roads to improve safe and efficient traffic flow on the road system.

Accessibility (a dimension of mobility)

Conceptually the ease in engaging in activities; ability to reach desired destinations, activities, goods, and services. Performance measures typically associated with this mobility dimension are:1.Time, distance, or cost to reach a destination; 2.Modal choices/alternatives; 3.Connectivity; and 4.Number of transfers (transit).

Action Plan

The initial consideration of the engineering design principles to be applied to the facilities of corridor segments. Action Plans provide detailed planning guidance for the project development process of controlled access facilities that make up corridors or corridor segments by establishing the basis for the Purpose and Needs Statement. The plans identify preliminary typical sections for facilities in the corridor segment, define the controlling design criteria, such as design speed for a highway, and identify multimodal opportunities.


One of the values for the Spatial Context Flow Characteristic. Data that is hyper local (relevant to a geographic area within ~1 minute travel distance).


These are the operators that set control parameters, implement system policies, monitor system configuration, and make changes to the system as needed.

Advance Acquisition

The acquisition of real property rights for use on a transportation corridor in advance of the fiscal year in which right of way acquisition would normally occur. This is done to take advantage of favorable prices or the availability of land and to preclude further development that would make the property more costly to the public.

Aftermarket Safety Device

A connected device in a vehicle that operates while the vehicle is mobile, but which is not connected to the data bus of the vehicle.


The process of combining data elements of similar format into a single data element that is a statistical representation of the original elements.

Allowed Movements

In the context of connected vehicle, the directions of movement that are legally allowed at a specific point in time based upon the state of the intersection signals.

Alternative Modes

Non-highway surface modes, such as rail, transit, walking and bicycling.


The process of studying a system by partitioning the system into parts (functions, components, or objects) and determining how the parts relate to each other.

Annual Average Daily Traffic (AADT)

The total volume of traffic on a highway segment for one-year, divided by the number of days in the year.

Annual Average Daily Truck Traffic (AADTT)

The total volume of truck traffic on a highway segment for one-year, divided by the number of days in the year.


Lacking individuality, distinction, and "recognizability" within message exchanges.


One or more pieces of software designed to perform some specific function. For example, a software program with an interface, enabling people to use a computer as a tool to accomplish a specific task.

Application Protocol Data Unit

Application Protocol Data Unit (APDU) is a defined data structure that is transferred at a peer level between two applications.

Application User

A user who interfaces with Application Layer software for a desired function or feature.


Fundamental concepts or properties of a system in its environment embodied in its elements, relationships, and in the principles of its design and evolution. 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 Context and Scope

Description of purpose, environment, and relevant concerns of an architecture. Every architecture description should have associated with it information that describes the purpose of the system(s) described, the environment in which it exists, identification of stakeholders, their perspectives and concerns, relevant disciplines and/or domains and a unique method for identifying the architecture.

Architecture Flow

See Information Flow

Architecture Interconnect

Communications paths that carry information between Physical Objects (subsystems and terminators) in the physical view of ARC-IT. Several different types of interconnects are defined in ARC-IT 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 view. The following types of communications links are defined: Center to Center (C2C), Center to Field (C2F), Field to Field (F2F), Wide Area Wireless (WAW), Short Range Wireless (includes Dedicated Short Range Communications or DSRC), Human Interface (e.g., what the system user sees and hears), Vehicle On-Board, Contact or Proximity, Wide Area Broadcast, Position Location Interface, Network Time Protocol, and Personal Area Network.


A judgment about unknown factors and the future which is made in analyzing alternative courses of action.

Asymmetric Encryption Algorithm

A cryptographic algorithm that uses two related keys, a public key and a private key, such that the public key is derived from the private key but, given only the public key, it is computationally infeasible to derive the private key.


A cryptographic service that provides assurance that the sender of a protocol data unit (PDU) is who they claim to be.


The quality of being genuine or authentic; which is to have the origin supported by unquestionable evidence; authenticated; verified. This includes whether the software or hardware came from an authorized source.


1) A cryptographic service that provides assurance that the sender of a protocol data unit (PDU) is entitled to certain permissions.

2) The process of determining the types of activities or access is permitted to an individual user, either human or system.

Authorization Certificate

A certificate that is used to validate application protocol data units (PDUs) other than certificate requests.

Auto (automobile)

A highway travel mode that includes motor vehicle traffic, including motorcycles, passenger cars, and four tire single units (FHWA Vehicle Category Classification, Classes 1- 3).


A combination of the auto and truck modes (FHWA Vehicle Category Classification Classes 1-13).

Automated Driving System

The hardware and software that are collectively capable of performing the entire DDT on a sustained basis, regardless of whether it is limited to a specific operational design domain (ODD).

Automated Vehicle

Vehicle integrated with an ADS.

Automobile (auto)

A travel mode that includes all motor vehicle traffic using a roadway except transit buses (includes such vehicles as trucks, recreational vehicles, motorcycles and tour buses) (HCM definition).


Ensuring timely and reliable access to and use of information where a loss of availability is the disruption of access to or use of information or an information system.

Average travel speed (for auto/truck modes)

The length of the highway segment divided by the average travel time of all vehicles traversing the segment, including all stopped delay times.