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Page 63
Suggested Citation:"Glossary." National Academies of Sciences, Engineering, and Medicine. 2011. Potential Safety Benefits of Motor Carrier Operational Efficiencies. Washington, DC: The National Academies Press. doi: 10.17226/14612.
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Page 63

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63 Benchmarking—To compare company practices and out- comes to those of other carriers (external benchmarking), or to track them in relation to past performance or to goals (internal benchmarking). Correlation—The degree of association or predictability between two variables (e.g., height and weight) among the same group of subjects (e.g., drivers). Correlation coefficient—A statistic summarizing direction and degree of association. Correlation coefficients range from −1.0 (a perfect inverse relation) through zero (no sta- tistical association) to +1.0 (a perfect linear relation). Critical Reason (CR)—In the LTCCS, the human, vehicle, or environmental failure leading to the Critical Event and thus to the crash. The immediate or proximal cause of a crash. Deadheads—Empty backhaul trips. Detention—Excessive driver delays associated with truck loading and unloading. Diversion—The practice of avoiding freeway tolls by choosing alternative, untolled routes, which often are undivided high- ways with more traffic interaction and higher crash risks. Exposure—Vehicle-miles traveled (VMT), hours driving, or other denominator to determine crash rates. Exposure data are essential for determining relative risk for different drivers, vehicle types, and driving situations. Haddon Matrix—A framework for understanding and design- ing crash reduction strategies. The 3 × 3 matrix juxtaposes time frame (i.e., pre-crash, crash, and post-crash) and agent (i.e., human, vehicle, environment). Expansions of the Haddon Matrix account better for the complexities of CMV transport. Higher-Productivity Vehicles (HPVs)—Vehicles with Gross Vehicle Weight Ratings (GVWRs) of more than 80,000 lb, the maximum size of standard tractor-semitrailers; includes Longer Combination Vehicles (LCVs). Likert scale—Common survey technique in which answer choices are presented as numeric rating scales, often with five choices numbered from 1 to 5. Mean—The arithmetic average score in a group of scores, computed by adding all the scores and dividing the sum by the number of cases. Median—The middle score in a group of scores. The point or score that divides the group into two equal parts. The median is also known as the 50th percentile. Naturalistic driving—Safety research in which vehicles are instrumented with video camera and various dynamic sen- sors. Subjects are fully informed and usually paid, but they quickly revert to driving in their normal manner. This per- mits observation of driving behavior and traffic events as they naturally occur. Navigation—Way-finding, generally in the context of a par- ticular A-to-B trip. Odds ratio—A statistic often used to quantify relative risk or occurrence of an outcome for two different situations or groups. An odds ratio greater than 1.0 implies over- involvement (e.g., in driving incidents), whereas an odds ratio less than 1.0 implies under-involvement. Response bias—The tendency, likely strong in the current surveys, for respondents to be more committed and inter- ested in the topic than those not responding. Because of response bias and other factors, the surveys in this project should not be considered representative of larger groups (e.g., all motor carrier safety managers). Risk avoidance—In this report, refers to operational practices that deploy vehicles and drivers efficiently and with safety benefits; includes safety-conscious routing (e.g., maximizing freeways, minimizing peak hour driving, assigning familiar routes) and similar deployment strategies. Risk factor—Any prior factor (driver, vehicle, environment, carrier) that affects the probability of a crash. Risk reduction—In the context of this report, refers to con- ventional carrier safety efforts to improve the safety per- formance of individual drivers and vehicles. This usually involves making company investments in proven interven- tions such as improved driver selection, training, manage- ment oversight, or vehicle safety equipment; contrasted with risk avoidance, defined earlier. Routing optimization—Improvements in the efficiency of an overall pickup and delivery sequence, as in a full driver tour-of-duty or multiday trip (Bennett 2009). Speed limiters—Electronic controls that limit the top pow- ered speed of vehicles; also called speed governors. Speed paradox—Although excessive speed is the biggest single proximal cause of crashes, there is generally less crash risk at higher travel speeds across the normal ranges of speed (i.e., not including overspeeding). The speed para- dox demonstrates the overall positive association between travel efficiency and safety. Telematics—General term encompassing onboard sensors, networks, software, GPS, and wireless communications that are becoming commonplace in today’s commercial vehicles. GLOSSARY

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TRB’s Commercial Truck and Bus Safety Synthesis Program (CTBSSP) Synthesis 20: Potential Safety Benefits of Motor Carrier Operational Efficiencies addresses risk avoidance strategies and highlights their use and perceived safety effects. The report is designed to assist motor carriers in deploying their vehicles in ways that may minimize crash risk.

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