Glossary of Terms
Area type — type of area (urban, suburban, or rural) for which the CMF was developed
Crash Modification Factor — multiplicative factor used to compute the expected number of crashes after implementing a given countermeasure
Crash Modification Function — produces a multiplicative factor used to compute the expected number of crashes after implementing a given countermeasure. The factor can vary for different scenarios, such as for different traffic volume scenarios.
Crash Reduction Factor — estimate of the percentage reduction in crashes due to a particular countermeasure
Crash Reduction Function — produces an estimate of the percentage reduction in crashes due to a particular countermeasure. The factor can vary for different scenarios, such as for different traffic volume scenarios.
Crash severity — severity of crashes which will be affected by the implementation of the particular countermeasure
Crash type — type of crashes which will be affected by the implementation of the particular countermeasure
Intersection geometry — configuration(s) of intersection (3-leg, 4-leg, 4 or more legs) at which the particular CMF was developed. The CMF value only applies to intersections of this geometric configuration.
Intersection type — type of intersection (3-leg, 4-leg, 4 or more legs) at which the particular CMF was developed. The CMF value only applies to intersections of this type.
Number of lanes — number of total through lanes for roadways at which the particular CMF was developed. The CMF value only applies to roadways with this many lanes.
Prior condition — condition of the road or intersection before countermeasure was implemented
Roadway type — type of roadway at which the particular CMF was developed. The CMF value only applies to roadways of this type. This list of roadway types was adapted from the Highway Performance Measurement System (HPMS) list of roadway types.
Sample size — number of observations on which the study developed the CMF
Sample size, after — number of observations in the time period after the countermeasure was implemented (applies to before/after studies)
Sample size, before — number of observations in the time period before the countermeasure was implemented (applies to before/after studies)
Sample size, required — number of observations that would be required, based on the study methodology, in order to get a statistically significant result
Search Area — fields which will be searched for the search terms. You may wish to use these checkboxes to narrow your search. All fields are searched by default, so specifying which field is to be searched may assist in getting more specific results. For example, if you are searching for countermeasures involving median installation using the search term "median", you will get search results from CMFs that have the word "median" in the countermeasure name, study title, study citation, abstract, and several other fields. This may produce search results that do not have to do with median installation. However, if you check the box for "countermeasure name", the search results will only contain CMFs that have the word "median" in the countermeasure name, resulting in a smaller set of results.
Standard error, unadjusted — estimate of the standard deviation of the CMF, which indicates how precise the CMF is. The unadjusted value is either the value reported in the paper/report or computed based on the information provided in the paper/report
Standard error, adjusted — product of the unadjusted standard error with the method correction factor (MCF). MCF is a parameter that is related to the study type and quality, and varies between 1.2 and 7. This parameter is assigned to each study based on a procedure described in the Highway Safety Manual.
Star rating — quality rating (1, least reliable, to 5, most reliable) that indicates the quality or confidence in the results of the study that produced the CMF
Study type — type of methodology used to produce the CMF
Traffic control — type of traffic control at intersections at which the particular CMF was developed. The CMF value only applies to intersections of this type. This value refers to the traffic control present before the countermeasure was implemented, in cases where the countermeasure affects traffic control.
Traffic volume — amount of traffic going both ways on the roadway
Years of data — years of data used in the study that produced the CMF.
Access management — relates to managing access to the roadway, including medians, left turn restricting designs such as left-overs, and driveway reduction
Advanced technology and ITS — relates to technology-driven strategies, including such things as red light cameras, speed cameras, and dynamic warning signs
Alignment — relates to vertical or horizontal alignment of the roadway, including such things as grade, curve radius, and spirals
Highway lighting — relates to lighting along the roadway
Interchange design — relates to interchange design, including such things as conversion to another type of interchange, ramp design, and acceleration/deceleration lanes
Intersection geometry — relates to geometric and physical design of an intersection, including such things as adding turn lanes, channelization, and conversion to other intersection types such as roundabouts
Intersection traffic control — relates to traffic control at intersections, including such things as signals, stop signs, and signal phasing
On-street parking — relates to parking on the street, including such things as prohibitions, time of day restrictions, and parking design
Pedestrians and bicyclists — relates to pedestrian or bicycle safety
Railroad grade crossings — relates to railroad grade crossings, including such things as signals, gate arms, and warning devices
Roadside — relates to anything beyond the shoulder, including such things as slopes, ditches, culverts, abutments, and guardrails
Roadway — relates to the traveled surface of the roadway, including all types of lanes (through, turning, passing), and the roadway surface
Roadway signs and traffic control — relates to signs and traffic control on roadway segments (as opposed to intersections)
Roadway delineation — relates to delineation of the road and travel lines, including such things as lines and pavement markers
Shoulder treatments — relates to anything on the paved or unpaved shoulder of the roadway, including such things as shoulder widening, shoulder barriers, and shoulder rumble strips
Speed management — relates to the management of vehicle speeds
Work zone — relates to work zones, including such things as lane closures, times of activity, and traffic operations
Definitions of Study Methodology Types
Regression cross section studies typically involve the estimation of regression models where the dependent variable is either crash frequency or crash rate and the independent variables may include site characteristics such as traffic volume, lane width, shoulder width, number of turn lanes (for intersections), etc. These studies typically include data from sites with and without the specific treatment under consideration.
Non-regression cross-section studies involve the direct comparison of crash frequency or crash rate between two groups of sites, one group with the treatment, and the other group without the treatment.
Case control methods are very common in the epidemiology field where this method is used to relate risk factors within a study population to a particular outcome or disease. In the highway safety context, the "outcome" is defined as a crash, the "risk factor" is a particular geometric feature or countermeasure and the "subjects" are roadway segments or intersections. In safety studies, "cases" could be defined as those entities that experience at least one crash during a particular time period and "controls" are similar entities that do not experience any crashes during the same time period. Odds ratios are estimated to examine the safety effectiveness of treatments.
In a simple before/after study, the crash frequency before the implementation of a treatment is compared with the crash frequency after the implementation of a treatment. Many simple before/after studies do not explicitly account for changes in traffic volume during the study period and also do not account for possible bias due to regression-to-the-mean (RTM) and other trends during the study period.
Before/after using empirical Bayes or full Bayes
Before/after using empirical Bayes or full Bayes methods are considered the state-of-the-art for doing observational before/after studies. Similar to the simple before/after study, crash frequency before the implementation of a treatment is compared with the crash frequency after the implementation of a treatment. However, these studies make use of a reference and comparison group to account for possible bias due to regression-to-the-mean (RTM), traffic volume, and other trends during the study period.
Meta-analysis combines the results of multiple studies to come up with a combined estimated of the safety effectiveness of a treatment.