Glossary

Average Annual Daily Traffic (AADT)

The total traffic for a year divided by 365.

Average Daily Traffic (ADT)

The total traffic volume during a given period (from 1 to 364 days) divided by the number of days in that period. Current ADT volumes can be determined by continuous traffic counts or periodic counts. Where only periodic traffic counts are taken, ADT volume can be established by applying correction factors such as for season or day of week. For roadways having traffic in two directions, the ADT includes traffic in both directions unless specified otherwise.

Congestion Management System (CMS)

A systematic process which provides information on transportation system performance and alternative strategies to alleviate congestion and enhance the mobility of persons and goods. A CMS includes methods to monitor and evaluate performance, identify alternative actions, access and implement cost-effective actions, and evaluate the effectiveness of implemented actions.

Congestion Mitigation and Air Quality (CMAQ) Improvement Program

A special provision of the Intermodal Surface Transportation Efficiency Act that directs funds towards projects in Clean Air Act Non-Attainment areas for ozone and carbon monoxide.

Congestion Pricing

The policy of charging drivers a fee that varies with the level of traffic on a congested roadway. Congestion pricing is designed to allocate roadway space in a more efficient manner. Congestion pricing is also known as relief tolling, variable pricing, and road pricing.

Design Hour Volume (DHV)

Typically, the DHV is the thirtieth highest hourly traffic volume for the design year, commonly twenty years from the time of construction. The DHV is given in units of vehicles per hour.

Directional Design Hour Volume (DDHV)

Typically, the DDHV is the thirtieth highest hourly traffic volume in one direction for the design year, commonly twenty years from the time of construction. DDHV is given in units of vehicles per hour.

Environmental Protection Agency (EPA)

A Federal agency charged with protecting the natural resources on the nation.

Feasibility Study

A study about a project's feasibility which is summarized in a document. The study addresses issues including the project's benefits, costs, effectiveness, alternatives considered, analysis of alternative selection, environmental effects, public opinions, and other factors. The feasibility study for major projects involving Federal funds was replaced under the Intermodal Surface Transportation Efficiency Act by the Major Investment Study.

Federal Highway Administration (FHWA)

The federal agency responsible for the administration of federal highway funds. FHWA does not have a direct role or responsibilities in the development of urban transportation plans or their development. However, their role is administration of federal funds and in the issuance of policy and procedure timetables for implementation of federal legislative directives is immense.

Federal Transit Administration (FTA)

The FTA was formerly as the Urban Mass Transportation Administration. It operates under the authority of the Federal Transit Act. The FTA was established as a component of the US Department of Transportation.

Grade Separation

A vertical separation between intersecting roads or railroad tracks. One facility travels over the other via an overpass or other structure.

High Occupancy Vehicle (HOV)

Vehicles having more than one occupant. Examples include carpools, vanpools, buses, and mini-buses. Transportation systems may encourage HOV use by having designated HOV lanes.

High Occupancy Vehicle (HOV) Lane

High Occupancy Vehicle Lane. Exclusive road or traffic lane limited to buses, vanpools, carpools, emergency vehicles, and in some cases, single occupant motorcycles. HOV lanes typically have higher operating speeds and lower traffic volumes than adjacent general purpose lanes. HOV lanes have proven to be successful in major metropolitan areas across the US; however, their full effectiveness is usually not realized until about one to two years after implementation.

Hourly Volume (HV)

The volume of traffic (given in units of vehicle per hour) that traverses across a segment of a roadway in one hour. The HV may be determined from traffic counts or may be a projected calculation. Refer to Design Hour Volume.

Intelligent Transportation System (ITS)

ITS replaces the term Intelligent Vehicle Highway System. ITS is a surveillance system designed to monitor traffic flows on major freeways and to inform motorists or problem areas. Some ITS technology includes changeable message signs, cameras, and video detectors. ITS applications include: the integration of traffic control and transportation management systems; traffic signals which adapt to traffic and change control each cycle; highway advisory radio systems; changeable message signs (CMS); vehicle detectors; closed-circuit television; Global Positioning Systems and route guidance (currently used in some trucks, buses, and rental cars). An ITS may enable drivers to operate their vehicles more safely and with greater knowledge about existing traffic conditions.

Intermodal Surface Transportation Efficiency Act (ISTEA)

A Transportation Bill passed by Congress in 1991 that provides six year authorizations for development of a National Intermodal Transportation System which consists of all forms of transportation in a unified, interconnected manner. It increased the percentage of federal match funding available for the majority of Texas transportation projects. ISTEA was designed to move goods and people more efficiently by developing an integrated transportation infrastructure system for the United States. ISTEA is this country's first serious effort to promote intermodalism, the idea that all systems and structures for moving people and goods must function as an integrated and cohesive transportation network which includes rail and mass transit as well as highways. Under ISTEA, the statewide planning process must incorporate some new goals: to reduce congestion and improve air quality; to consider national and international commerce; to consider energy conservation; to create an integrated system of several modes; and to concentrate on the most efficient way to move goods and people, not just people. The three major components of ISTEA are the National Highway System, the Surface Transportation Program, and the Congestion Mitigation and Air Quality Improvement Program.

K-Factor

A factor which represents the proportion of Average Annual Daily Traffic (AADT) expected to occur in the design hour. The K-factor is also known as the design hour factor and is normally expressed in design problems as DHV=AADT x K, where the DHV is the total two-way Design Hour Volume, and K is estimated from the ratio of the 30th Hourly Volume to the AADT from a similar site.

Level Of Service (LOS)

A qualitative measure describing operational conditions within a traffic stream and motorists' perceptions of those conditions. For example, LOS A represents free flow, almost complete freedom to maneuver within the traffic stream. LOS F represents forced flow, more vehicles are attempting to use the freeway than can be served resulting in stop and go traffic.

Metropolitan Planning Organization (MPO)

An association of local agencies established for mutual benefit and to help coordinate planning and development activities within a metropolitan region. Establishment of the MPO is required by law in urban areas of over 50,000 population if federal funds are to be used. The MPO consists of two groups.

The Policy board is comprised of officials representing the counties, cities, and state agency.
The technical advisory group consists of professional planners and engineers who are usually employees of the same agencies.

The MPO is not a level of government. However, the MPO has "effective control" over transportation improvements within the area since a project must be a part of the MPO's adopted plan in order to receive federal funding.

National Highway System (NHS)

The NHS is one of the three major components of the 1991 Intermodal Surface Transportation Efficiency Act. A major new Federal-aid system was established in the NHS. It includes the Interstate System, other routes identified as having strategic defense characteristics, routes providing access to major ports, airports, public transportation and intermodal transportation facilities and, of particular significance to local governments, and many principal urban and rural arterials which provide regional service.

Non-Attainment Areas

These are geographical areas, defined by the Environmental Protection Agency, whose air quality does not meet Federal air quality standards designed to protect public health.

Peak Hour Factor (PHF)

The PHF is a ratio of the total hourly traffic volume to the maximum 15-minute traffic rate within the hour. PHF represents the flow variation within an hour. Observations consistently show the flow rate found in the peak 15-minute period within an hour is not sustained throughout the entire hour. PHFPHFs

Peak Hour Traffic (PHT)

Higher traffic volumes occur in the evening and in the morning because of workrelated trips. The evening peak is typically the highest since non-work related trips are also made in this period. The percentage of Average Daily Traffic (ADT) which occurs in the evening peak hour commonly ranges between 7% and 12%.

Record Of Decision (ROD)

The final approval of an Environmental Impact Statement issued by the Federal Highway Administration.

Right Of Way (ROW)

The land (usually a strip) acquired for or devoted to transportation purposes. For example, highway ROW and railroad ROW.

Single-Occupant Vehicle (SOV)

A vehicle having only one occupant (usually the vehicle operator!).

Statewide Transportation Improvement Program (STIP)

A staged, multi-year, statewide, intermodal program of transportation projects which is consistent with the Statewide Transportation Plan (STP) and planning processes and metropolitan plans, TIPs and processes. The STP is to be implemented within each 3-year period after adoption of the program, and it includes a financial plan that demonstrates how the program can be implemented. Requirement for the STP was established by the Intermodal Surface Transportation Efficiency Act (ISTEA) of 1991. The STP is developed in cooperation with the Metropolitan Planning Organization (MPO) programs.

Statewide Transportation Plan (STP)

The official statewide, intermodal transportation plan that is developed through the statewide planning process.

Texas Department of Transportation (TxDOT)

A State agency responsible for planning, constructing, maintaining, inspecting, regulating, and distributing funding for state transportation facilities including roads, bridges, waterways, and airports (to a degree). TxDOT also provides technical assistance to counties. Formerly known as the State Department of Highways and Public Transportation (SDHPT). Unofficially known as the Highway Department.

Transportation Improvement Program (TIP)

The TIP is a staged, three- to five-year prioritized program of transportation projects covering a metropolitan planning area which is consistent with the metropolitan transportation plan. The projects are recommended from those in the transportation systems management element and the long-range element of the planning process. This program is required for a locality to receive federal transit and highway grants. The selected projects need to be consistent with the transportation plan. The TIP also contains an annual or biennial element which lists all transportation project activities that will receive federal funding for a given 1 or 2 year period. Regulations require the Metropolitan Planning Organization (MPO) and state and transit operators to cooperatively develop the TIP and make project selections within expected funding levels.

Transportation Management Area (TMA)

An urbanized area with a population over 200,000 (as determined by the latest decennial census) or other area when TMA designation is requested by the Governor and the Metropolitan Planning Organization (MPO), and officially designated by the Federal Highway Administration and the Federal Transit Administration. The TMA designation applies to the entire metropolitan planning area served by an MPO within which the TMA is located.

Unified Transportation Program (UTP)

A ten year planning document that guides and controls project development for the Texas Department of Transportation in a feasible and economical manner. The UTP is updated annually and requires approval by Texas Transportation Commission. Annual updating allows the UTP to be customized as the Intermodal Surface Transportation Efficiency Act (ISTEA) programs become better defined, and the annual updating also enables the UTP to serve as an integral part of the planning process required by ISTEA.

Vehicle Mile of Travel (VMT)

A unit to measure vehicle travel made by a private vehicle, such as an automobile, van, pickup truck, or motorcycle. Each mile traveled is counted as one vehicle mile regardless of the number of persons in the vehicle.

Volume-to-Capacity (V/C) Ratio

The ratio of flow rate to capacity. The V/C ratio may be the actual or projected rate of flow on a designated lane group during a peak 15-minute interval divided by the capacity of the lane group. The V/C ratio is a measure of capacity sufficiency, that is, whether or not the physical geometry provides sufficient capacity for the subject movement. Low V/C ratios depict relatively free flow conditions. High V/C ratios depict more congested conditions. Actual V/C ratios are calculated from vehicle count data (defining volume) and the geometrics of a roadway (determining capacity). V/C ratios are used to broadly define problem areas on a freeway and to make preliminary operational decisions concerning the freeway (e.g., ramp metering rates). In some cases, the V/C ratio is used to define Level of Service.

Weaving Area

A section of a highway where two or more vehicle flows must cross each other's path along a length of the freeway. Weaving areas are usually formed when merge areas are closely followed by diverging areas. They are also formed when a freeway on-ramp is followed by an off-ramp and the two are connected by a continuous auxiliary lane.