| Term |
Definition |
Source |
| Areas |
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 do not need to be parallel to each other. |
2016 HCM(18) |
| Areawide |
A generic term that includes all geographic scales that are not facility specific, such as neighborhood, network, system, region, city, State, etc. |
Research team |
| Corridors |
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. |
2016 HCM(18) |
| Crash frequency (also called “Crash frequency—count” in this document) |
The number of crashes occurring at a particular site, facility, or network in a 1-yr period. Crash frequency is measured in number of crashes per year. |
2016 HCM(18) |
| Crash frequency—density |
The number of crashes occurring per unit area (e.g., square miles), unit length (e.g., mile), or area (such as an established U.S. Census geographic area of block, block group, or tract or established geographic regions such as city, county, or metropolitan statistical area). |
Research team |
| Crash rate |
The number of crashes per unit of exposure. For an intersection, this is typically the number of crashes divided by the total entering AADT; for road segments, this is typically the number of crashes per million VMT on the segment. |
2010 HSM(2) |
| Exposure |
The measure of the number of potential opportunities for a crash to occur. This theoretical definition has been quantified or estimated numerous ways in practice. |
Research team |
| Exposure scale |
The most granular geographic level for which an exposure measure is desired. |
Research team |
| Facilities |
The 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. |
2016 HCM(18) |
| 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.(35) |
Research team |
| Points |
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). |
2016 HCM(18) |
| Region |
A geographic scale (mentioned in the original FHWA Statement of Work) that is most comparable to the term “system” as defined in the 2010 HCM.(35) |
Research team |
| Risk |
The 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. |
Research team |
| Risk factor |
Any attribute or characteristic that increases the likelihood of a negative safety outcome (e.g., crash or fatality). |
Research team |
| Segment |
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). |
2016 HCM(18) |
| System |
All the transportation facilities and modes within a particular region. |
2016 HCM(18) |
| Safety index |
A measure to minimize known biases in the analysis procedure. |
Research team |
