The National Academies Press

Currently Skimming:

Pages 123-137

The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.

From page 123... ... 123 This appendix provides an overview of quantitative safety performance, explains how quantitative safety analysis can inform decisions, introduces the general methods for quantifying safety performance, and presents a decision process for selecting an appropriate method. Introduction to Quantitative Safety Analysis One of the most basic components of evidence-based safety is quantifying the safety performance of an existing or planned facility in terms of the estimated frequency and severity of crashes. Read the entire page →
From page 124... ... 124 Application of Crash Modification Factors for Access Management • A proposed alternative that incorporates some changes to the roadway geometrics and/or operations. The typical measure of safety performance is the estimated number of crashes, but this could be in terms of specific crash types or severities. Read the entire page →
From page 125... ... Overview of Quantitative Safety Analysis 125 provides a larger sample of crashes for analysis; however, a longer study period also increases the chances for other changes over time such as safety and operational improvements; natural degradation of surface friction and sign reflectivity; and changes in general land use and traffic patterns. As such, it is important to balance the length of the study period with the potential for other changes over time. Read the entire page →
From page 126... ... 126 Application of Crash Modification Factors for Access Management Figure B-2. SPF for multivehicle crashes at four-legged signalized intersections on urban and suburban arterials with major road traffic volume of 10,000 vehicles per day. Read the entire page →
From page 127... ... Overview of Quantitative Safety Analysis 127 observed crashes is zero and the expected number of crashes is based solely on the predicted crashes. If a reliable SPF is not available for the facility type of interest and the crash history is available and applicable, then the weight assigned to the SPF is zero and the expected number of crashes is based solely on the observed crashes. Read the entire page →
From page 128... ... 128 Application of Crash Modification Factors for Access Management Crash Modification Factors and Functions Crash modification factors and functions (CMFs) can be applied to observed, predicted, or expected crashes to estimate the change in safety performance associated with changing the geometric or operational conditions of the roadway. Read the entire page →
From page 129... ... Overview of Quantitative Safety Analysis 129 subject site on at least a few, if not many, characteristics. To reasonably assume the CMF value is applicable to the subject site, it should match the subject site on site characteristics that are known to have a statistically significant influence on the value of the CMF such as traffic volume, number of intersection approaches, and number of lanes. Read the entire page →
From page 130... ... 130 Application of Crash Modification Factors for Access Management CMFs should only be applied within the context for which they were developed. For example, Table B-1 presents CMFs related to corner clearance where the base condition is no driveways present within 50 feet of the downstream corner. Read the entire page →
From page 131... ... Overview of Quantitative Safety Analysis 131 In some cases, multiple strategies are applied to a single location, such as signalizing a stopcontrolled intersection, adding turn lanes, and relocating or consolidating driveways. Each of these strategies may have a separate CMF, or a single CMF may represent the safety impact of the combined strategies. Read the entire page →
From page 132... ... 132 Application of Crash Modification Factors for Access Management or driver behavior, all of which could impact the safety performance of the facility. It solely uses past site crash history as the basis for estimating future safety performance. Read the entire page →
From page 133... ... Overview of Quantitative Safety Analysis 133 Predicted Crashes Table B-3 provides a summary of the strengths and limitations of using predicted crash frequency to estimate long-term safety performance. This approach assumes the SPF prediction is representative of future safety performance. Read the entire page →
From page 134... ... 134 Application of Crash Modification Factors for Access Management Decision Process It is important to understand the strengths and limitations of these approaches because the reliability of the approach will impact the reliability of analysis results and decision-making. In selecting an appropriate analysis approach to estimate safety performance, there is a need to consider the level of rigor, scope of the project, and availability of data. Read the entire page →
From page 135... ... Overview of Quantitative Safety Analysis 135 2. Predicted crashes are typically more reliable than observed or expected crashes if the site characteristics change drastically from the past condition to the present or future condition of interest. Read the entire page →
From page 136... ... 136 Application of Crash Modification Factors for Access Management Summary Quantitative safety performance can help inform decision-making processes. Safety performance is defined by the estimated frequency and severity of crashes. Read the entire page →
From page 137... ... Overview of Quantitative Safety Analysis 137 • Expected Crashes: Expected crashes are based on a weighted average of the observed and predicted crashes through use of the EB method. It accounts for annual fluctuations in observed crashes and changes in traffic volume, roadway characteristics, and general time trends, while also considering historical crash data. Read the entire page →

From page 123...

... 123 This appendix provides an overview of quantitative safety performance, explains how quantitative safety analysis can inform decisions, introduces the general methods for quantifying safety performance, and presents a decision process for selecting an appropriate method. Introduction to Quantitative Safety Analysis One of the most basic components of evidence-based safety is quantifying the safety performance of an existing or planned facility in terms of the estimated frequency and severity of crashes.