Modeling Glossary

The following is a listing of Modeling-terms developed by CCMPO. Acronyms are listed by letter, but are not in alphabetical order.

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Term	Acronym	Definition
Buildout Analysis		A form of analysis predicting the total amount of development that could possibly occur in a given area under existing or proposed legal constraints (e.g. zoning ordinance) and environmental constraints (e.g. wetlands, floodplains, steep slopes, etc.).
Calibration		The process of developing the model parameters using observed transportation data (e.g. traffic counts). Model calibration optimizes the agreement between observed data and the model’s predicted data.
Capacity		A transportation facility's (road, railway, etc.) ability to accommodate a moving stream of people or vehicles in a given time period.
Congestion		A condition which hinders movement on a transportation facility at optimal legal speeds. Frequently characterized by unstable traffic flows.
Congestion Management Process	CMP	Formerly known as a Congestion Management System (CMS), CMP is a federally-mandated program within Metropolitan Planning Organizations to address and manage congestion.
Decision-Support System	DSS	Computer assessment tool that examines the relationships between land use and transportation.
Design Hour Volume	DHV	Commonly (but, not strictly) the 30th highest hourly traffic volume for a given year. DHV has been considered to be an optimal traffic volume estimation for designing future transportation infrastructure (e.g. intersection and roadway capacity analysis, bridge design, and geometric specifications, et al.) since the 1950 release of the Highway Capacity Manual (HCM).
Destination		The place or zone in which a trip terminates.
Destination Choice		An estimate of a trip endpoint determined by a combination of the purpose and origin of the trip. Destination choice is believed to depend on characteristics of the individual (income, auto ownership), characteristics and location of activities at which the trip’s purpose can be accomplished, and characteristics of transportation modes connecting the origin to each possible location.
Geographic Information Systems	GIS	A software system which assists in the development, storage, analysis, and display of locational or spatial information.
Geographic Information Systems for Transportation	GIS-T	Refers to the principles and applications of applying geographic information technologies to transportation problems (Miller and Shaw, 2001).
Gravity Model		An underlying premise of Travel Demand Forecasting/Modelling, based on Sir Issac Netwon's "Law of Universal Gravitation". Newton's gravity model was converted into a geographical context by W.J. Reilly in 1933. This theory is still known today as "Reilly's Law". This law has two concepts: 1. That a city attracts most of its commuters from the closest towns. 2. That a larger city attracts larger numbers of commuters than a smaller city. "A city will attract retail trade from a town in its surrounding territory, in direct proportion to the population size of the city and inverse proportion to the square of the distance from the city." (Reilly, 1929).
Highway Capacity Manual	HCM	Published by the Transportation Research Board (TRB), the HCM outlines fundamental information and computational techniques on the quality of service and capacity of highway facilities.
Home-Based Trip		A trip that starts and/or ends at home.
Home-Based Work Trip		A trip with one end at work and the other at home.
Induced Travel		Formally known as the "Traffic Inducement Effect", it is a highly debated concept in transportation planning, which postulates that improvements intended to increase highway capacity are only a temporary solution. Simply stated, Induced Travel is any increase in daily travel (measured as Passenger Miles of Travel [PMT] or Vehicle Miles of Travel [VMT]) resulting from a change in the transportation system.
Intermodal		Planning that reflects a focus on connectivity between modes and emphasizes choices, coordination, and cooperation.
Journey-To-Work Data	JTW	Worker/Commuter home-based work trip sample data collected by the U.S. Census Bureau within the long form since 1960.
Land Use Allocation Module	LUAM	The Land Use Allocation Module (LUAM) has been used by CCMPO & CCRPC to forecast future land use patterns. The primary data inputs are; housing & employment by type of Traffic Analysis Zone (TAZ) in the model, physical constrains data, and zoning data.
Level of Service	LOS	A system of indicating delay at signalized intersections, which is graded on a letter scale from A to F, generally outlined by the HCM as: A <= 10 sec, B = 10-20 sec, C = 20-35 sec, D = 35-55 sec, E = 55-80 sec, F > 80sec.
Link		An element of a transportation network that represents a segment which terminates in a node at the other end. A link may have a number of attributes, including distances, number of lanes, capacity, and direction, and is often assigned a travel time function to the volume of traffic using the link.
Low Volume Road		According to the MUTCD (Section 5A.01), a low volume road is: "a facility lying outside of built-up areas of cities, towns, and communities, and it shall have a traffic volume of less than 400 AADT." According to the AASHTO "Green Book" (i.e. Guidelines for Geometric Design of Very Low-Volume Local Roads), a low volume road has an "ADT less than or equal to 400".
Mobile6		A vehicle emission modelling software, which develops factors for predicting gram per mile emissions of Hydrocarbons (HC), Carbon Monoxide (CO), Nitrogen Oxides (NOx), Carbon Dioxide (CO2), Particulate Matter (PM), and toxics from mobile sources under various conditions. This model has been upgraded/replaced by a model called MOVES.
Mode		One of several major types of transportation such as rail, bus, automobile, and non-motorized (bicycle/pedestrian) transportation.
Mode Choice		A process by which an individual selects a transportation mode for use on a trip chain, given the trip’s purpose, origin, and destination; characteristics of the individual; and characteristics of travel by the realistically available modes.
Mode Split		The percentage, or share, of trips captured by the various transportation modes.
Model		In Transportation or Land-Use Planning, a computer model is employed to generate forecasts on future conditions of population levels, traffic, economic development, housing development, etc. Such a tool ought to be considered a Decision-Support system, augementing sound judgement of a decision-making entity.
Motor Vehicle Emission Simulator	MOVES	EPA's Office of Transportation and Air Quality (OTAQ) is developing the "MOtor Vehicle Emission Simulator" (MOVES). The successor to MOBILE6, this more recent emission modeling system estimates emissions for on-road and nonroad mobile sources, covers a broader range of pollutants, and allows multiple scale analysis from fine-scale analysis to national inventory estimation. MOVES is planned to serve as the replacement for MOBILE6 and NONROAD for all official analyses associated with regulatory development, compliance with statutory requirements, and national/regional inventory projections.
Multimodal		Using several integrated modes of transportation (rail or bus transit, carpool, bicycle, pedestrian, single-occupancy vehicle, etc.) in a given area.
National Household Travel Survey	NHTS	The NHTS is national survey of daily and long-distance travel. The survey includes demographic characteristics of households, people, vehicles, and detailed information on daily and longer-distance travel for all purposes by all modes. Survey data are collected from a sample of U.S. households and expanded to provide national estimates of trips and miles by travel mode, trip purpose, and a host of household characteristics. NHTS updates information gathered by two series of travel surveys—the Nationwide Personal Transportation Survey (NPTS) conducted in 1969, 1977, 1983, 1990, and 1995 and the American Travel Survey (ATS) (covering U.S. Long-distance travel information), which was conducted in 1977 and 1995. The 2001-2002 NHTS combines questions about daily and long-distance trip-making characteristics in one survey. The seventh series of NHTS was undertaken in 2008-2009 to include approximately 150,000 households across the nation.
Network		In planning, a network is a computerized representation (often times in the for of a GIS or other digital model) of roadways and intersections that describes a transportation system. For highway engineering, a network is a configuration of highways that constitutes the total system. In transit operations (bus, passenger rail, et al.), a network is a system of transit lines or routes, usually designed for coordinated operation.
Node		A point where two links join a network, usually representing a decision point for route choice but sometimes indicating only a change in some important link attribute.
Off Peak		Trips that occur during period of relatively low traffic.
Origin		The location or zone at which a trip begins; the place where a trip is “produced.”
Origin/Destination Study	O-D	A study of where person or vehicle trips begin and end. Such a study may also include trip purposes and frequencies.
Passenger Car Equivalents	PCE	Traffic engineering/modelling practice of converting Trucks, Buses and RV's to cars for the purpose of Capacity or Level of Service (LOS) analyses. More information on this topic may be found in the Highway Capacity Manual (HCM).
Peak Hour Factor	PHF	PHF is used to evaluate roadway capacity. It is the ratio of of total hourly volume to the maximum 15 minute rate of flow within the hour, that is; PHF = Hourly Volume / Peak Rate of Vehicle Flow (within the hour).
Peak Period/Peak Hour		The time period (or specific hour) during which the roadway carries the greatest number of vehicles. Traffic impacts are typically evaluated during the morning and afternoon peak hours when the greatest numbers of motorists are traveling to and from work. It may be specified as the morning peak period (traditionally 7-9 AM) or evening (4-6 PM) peak.
Peak Spreading		A Travel Demand Forcasting modeling concept where there is a calculation of a time-of-day shift in the peak period (i.e. when the maximum amount of traffic occurs). Where many travel demand models use only PM peak hour data (anywhere from 4-6 PM), a peak-spreading travel demand model uses an entire day of data (e.g. a "Day-based model" as opposed to a "PM peak model") to calculate this figure.
Pedestrian Environmental Factor	PEF	In traffic modeling, the PEF identifies the pedestrian and bicycle-friendly parts of the transportation network.
Person Trip		The movement of a person from an origin to a destination. A carpool carrying three people from origin-to-destination make one vehicle trip, its occupants together have made three person trips.
Route Choice		The process of simulating the sequence of roadways an individual will choose for a trip, given the trip’s origin, destination, and mode.
Single Occupancy Vehicle	SOV	Vehicle carrying only a driver.
Special Trip Generators		In modeling context, special generators represent employers (or employer locations) with unique characteristics that are especially large and therefore need to be handled outside of the normal trip generation approach. Examples for the Chittenden County, VT model include; IBM (Essex and Williston), the University of Vermont, and Fletcher Allen Health Care.
Traffic Analysis Zone	TAZ	Chittenden County is divided into 335 small areas. Each area has relatively similar characteristics within its boundaries. Often derived from Census Blocks, or Block Groups, TAZs represent land use data on housing and employment in the Travel Demand modeling process.
Traffic Assignment		In the four step practice of Travel Demand Modeling, Traffic Assignment is the process of predicting the specific path travelers take from their Origin to their Destination.
TransCAD		A computer software “platform” used for the CCMPO transportation model.
Travel Demand Forecasting (Modelling)	TDF	A four-step (some consider it a five-step) process used to predict future traffic on a transportation network. The five steps, in order, are; 1. Trip Generation, 2. Trip Distribution, 3. Mode Split/Mode Choice, 4. Traffic Assignment, and 5. Model Validation and Reasonableness Checks.
Trip Attraction		The process of attracting trips to a zone. It is generally a function of the land uses in a zone.
Trip Distribution		The process of determining trip exchanges; the number of trips between each pair of zones.
Trip Generation		The first step in the four-step Travel Demand Modeling process, which estimates the number and types of trips types taken by travelers (e.g. Home-To-Work, Nonwork-To-Home, etc.).
Trip Generation Manual		Published by the Institute of Transportation Engineers (ITE), the Trip Generation Manual is intended to advise planners and engineers on trip generation rates based on land use category, building type, and other site plan related information. Trip generation rates have been developed from compiled traffic data collected from thousands of traffic studies. The Vermont Agency of Transportation (VTrans) released its own version of the Trip Generation Manual in 2010 in answer to the challenge that ITE trip generation rates are not representative of small urban and rural areas in Vermont.
Validation		The process of evaluating the accuracy of a transportation model.
Vehicle Hours of Delay	VHD	An indicator reflecting the time it takes to travel a roadway during peak periods v.s. the time it would to travel at a free-flow 35 miles per hour.
Vehicle Hours of Travel	VHT	Transportation performance measure that considers the amount of time trip-making takes.
Vehicle Miles Traveled	VMT	An indicator of occuring traffic, calculated using Annual Average Daily Traffic (AADT) multiplied by the length of a particular road in question. One car traveling one mile represents 1 unit of VMT.
Vehicle Trip		An Origin-to-Destination journey by a single vehicle. A bus carrying 40 people from an origin to a destination makes one vehicle trip, while its occupants make a total of 40 person trips.
Volume to Capacity Ratio	V/C	An indicator of congestion levels measured by the number of vehicles per hour (volume) divided by the maximum number of cars the road is designed to handle (capacity).

OTHER TRANSPORT TERMINOLOGY REFERENCES: