D. Glossary

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3R is a common term which refers to resurfacing, restoration, and rehabilitation projects.

4Es of Safety refers to the four major categories for addressing road safety; Engineering, Enforcement, Education, and Emergency Medical Services, which have typically been used either as measures to correct existing road safety issues or as crash prevention strategies.

Analysis period refers to a defined period of time for analysis.  Crash experience can vary at a location from year to year; therefore, it is important to use more than one year of data for the analysis; generally a minimum of three years is used.

Benefit/‌cost analysis is a quantitative measure commonly used in prioritizing projects and countermeasures which compares all of the benefits associated with a countermeasure (e.g., crash reduction, etc.), expressed in monetary terms, to the cost of implementing the countermeasure.

Crash Modification Factors (CMF) is a multiplicative factor used to compute the expected number of crashes after implementing a given countermeasure at a specific site.

Coefficient of variation is a normalized measure of dispersion of a probability distribution.  It is the ratio of the standard deviation to the mean value.

Cost-effectiveness is the amount of money invested divided by the benefit in crash reduction.

Crash Reduction Factors (CRF) is the percentage crash reduction that might be expected after implementing a given countermeasure

Data accessibility is a measure of how easy is it to retrieve and manipulate data in a system, in particular by those entities that are not the data system owner.

Data accuracy is a measure of how reliable the data are, and if the data correctly represent an occurrence.

Data completeness is a measure of missing information, including missing variables on the individual crash forms or underreporting of crashes.

Data integration is a measure of how well various systems are connected or linked.

Data timeliness is a measure of how quickly an event is available within a data system.

Data uniformity is a measure of how consistent information is coded in the data system, and/or how well it meets accepted data standards.

General Estimates System (GES) is a database of a national representative sample of police reported motor vehicle crashes of all types.  GES is directed by the National Center for Statistics and Analysis, which is a component of Research and Development in NHTSA.

Highway Performance Monitoring System (HPMS) is a national-level highway information system that includes data on the extent, condition, performance, use, and operating characteristics of the Nation’s highways.

Interactive Highway Safety Design Model (IHSDM) is a software analysis package that assists engineers with evaluating safety for two-lane rural highway design alternatives.

“KABCO” Injury Scale is frequently used by law enforcement for classifying injuries and also can be used for establishing crash costs.  (K – Fatal; A – Incapacitating injury; B – Nonincapacitating injury; C – Possible injury; and O – No injury.)

Overdispersion is a systematic variation in the number of accidents, whenever the variance exceeds the mean.  The amount of overdispersion of a data set is described in terms of the overdispersion parameter.

Net Present Value (NPV) is a method which expresses the difference between the discounted costs and discounted benefits of a safety improvement project.  The costs and benefits are “discounted” meaning they have been converted to a present value using a discount rate.  NPV also is referred to as net present worth (NPW).

Nominal safety refers to whether or not a design or design element meets minimum design criteria based on national or state standards and guidance documents such as the AASHTO Green Book and the MUTCD.  Nominal safety does not characterize the actual or expected safety of a roadway.

Quantitative analysis typically involves the identification and comparison of cost, effectiveness, and resilience (how long it is effective) for each countermeasure or program based on the latest research.

Regression to the Mean(RTM)describes a situation in which crash rates are artificially high during the before period and would have been reduced even without an improvement to the site.  Variations at a site are usually due to the normal randomness of crash occurrence.  Because of random variation, the extreme cases chosen in one period are very likely to experience lower crash frequencies in the next period – the highest get lower and the lowest get higher.

SafetyAnalyst is a set of software tools which utilizes SPFs for evaluating roadway locations and contains over 100 SPFs for various roadway segment types.  SafetyAnalyst includes modules for identifying locations for potential safety improvement, diagnosis and countermeasure selection, economic appraisal and priority ranking, and evaluation of implemented improvements.

Safety Performance Functions (SPF) are the change in the expected number of crashes as average daily traffic (ADT) or some other exposure measure increases, while all other factors affecting crash occurrence are held constant.

Statewide Transportation Improvement Program (STIP) is the financial programming document for the state representing a commitment of the projects and programs that will be implemented throughout the state using Federal-aid transportation and transit funding.

Substantive safety refers to the actual or expected safety on a roadway.

Transportation Improvement Program (TIP) is the programming document for metropolitan planning areas (urbanized areas with populations over 50,000) that identifies the projects and funding to be implemented to reach the vision for the metropolitan areas’ transportation system and services.

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