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FHWA Highway Safety Programs


Term Definition Source
Areas Areas consist of an interconnected set of transportation facilities serving movements within a specified geographic space, as well as movements to and from adjoining areas. The primary factor distinguishing areas from corridors is that the facilities within an area need not be parallel to each other. 2010 Highway Capacity Manual (HCM)
Areawide Generic term that includes all geographic scales that are not facility-specific, such as neighborhood, network, system, region, city, state, etc. This Guide
Census block The smallest entity for which the Census Bureau collects and tabulates decennial census information; bounded on all sides by visible and nonvisible features shown on Census Bureau maps. U.S. Census Bureau
Census block group A combination of Census blocks that is a subdivision of a census tract. U.S. Census Bureau
Census tract A small, relatively permanent statistical subdivision of a county in a metropolitan area or a selected nonmetropolitan county. Census tract boundaries normally follow visible features, but may follow governmental unit boundaries and other nonvisible features in some instances; they always nest within counties. Designed to be relatively homogeneous units with respect to population characteristics, economic status, and living conditions, census tracts usually contain between 2,500 and 8,000 inhabitants. U.S. Census Bureau
Corridors Corridors are generally a set of parallel transportation facilities designed to move people between two locations. For example, a corridor may consist of a freeway facility and one or more parallel urban facilities. 2010 HCM
Direct demand model A statistical model that estimates facility-specific pedestrian and bicyclist volumes based on observed volumes at a sample of locations and nearby context (such as land use and form, street type, etc.). Direct demand models are often based on regression analysis. NCHRP Report 770, Research Team
Exposure Measure of the number of potential opportunities for a crash to occur. This theoretical definition has been quantified or estimated many different ways in practice. This Guide
Exposure scale The granularity of the geographic level for which an exposure measure is desired. This Guide
Facilities Facilities are lengths of roadways, bicycle paths, and pedestrian walkways composed of a connected series of points and segments. The HCM defines freeway facilities, multilane highway facilities, two-lane highway facilities, urban street facilities, and pedestrian and bicycle facilities. 2010 HCM
Network A geographic scale (mentioned in the original FHWA Statement of Work) that is most comparable to the term Area as defined in the 2010 HCM. This Guide
Points Points are places along a facility where (a) conflicting traffic streams cross, merge, or diverge; (b) a single traffic stream is regulated by a traffic control device; or (c) there is a significant change in the segment capacity (e.g., lane drop, lane addition, narrow bride, significant upgrade, start or end of a ramp influence area). 2010 HCM
Region A geographic scale that is most comparable to the term System as defined in the 2010 HCM. This Guide
Risk Measure of the probability of a crash to occur given exposure to potential crash events. This theoretical definition has been quantified or estimated by dividing the expected or measured number of crashes by exposure. This Guide
Risk factor Any attribute or characteristic that increases the likelihood of a negative safety outcome (e.g., crash or fatality). This Guide
Segment A segment is the length of roadway between two points. Traffic volumes and physical characteristics generally remain the same over the length of a segment, although small variations may occur (e.g., changes in traffic volumes on a segment resulting from a low-volume driveway). 2010 HCM
Sketch planning Methods to estimate existing or future demand that are simpler alternatives to developing complex travel demand models.  Often, these methods are implemented in spreadsheets or geographic information systems (GIS) using existing travel survey and other data. This Guide
System Systems are composed of all the transportation facilities and modes within a particular region. 2010 HCM
Traffic analysis zone (TAZ) A common areawide geography that are defined by metropolitan planning organizations (MPO) for use in their travel demand forecasting models. TAZ are typically composed of multiple Census blocks. This Guide
Travel demand model A computerized process that estimates existing and future travel demand (often on a citywide or regional basis) given numerous inputs, such as the transportation network, population and demographic characteristics, and trip-making behavior. The end result of a travel demand model is traffic volume estimates on individual transportation network links. This Guide
Travel survey A systematic effort to collect information about individual travel behavior. Travel surveys are typically collected from a statistical sample of travelers for a specified day or days (not an entire month or year), and typically gather aggregate trip information (travel mode, trip purpose, trip start and end location, trip length or time, etc.). This Guide
Work trip Travel from home to work (also known as commuting). In their Journey to Work surveys, the U.S. Census Bureau collects trip information for only work trips. Trips that have a non-work purpose are collected by FHWA’s National Household Travel Survey (NHTS) and other regional household travel surveys (when administered). This Guide