National Academies Press: OpenBook
« Previous: Appendix B - Overview of Quantitative Safety Analysis
Page 138
Suggested Citation:"Appendix C - Calibration." National Academies of Sciences, Engineering, and Medicine. 2021. Application of Crash Modification Factors for Access Management, Volume 1: Practitioner's Guide. Washington, DC: The National Academies Press. doi: 10.17226/26161.
×
Page 138
Page 139
Suggested Citation:"Appendix C - Calibration." National Academies of Sciences, Engineering, and Medicine. 2021. Application of Crash Modification Factors for Access Management, Volume 1: Practitioner's Guide. Washington, DC: The National Academies Press. doi: 10.17226/26161.
×
Page 139

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

138 Calibration of the models involves the estimation and application of adjustment factors for applying the models in a jurisdiction and time period that are different from those used to develop the original models. The factors reflect differences in crash experience due to differences in terrain, climate, crash definition, and reporting. The following procedure, based on AASHTO’s Highway Safety Manual (1st edition), is suggested for approximating calibration factors, recog- nizing that there will be limitations in availability of the data required for more thorough procedures (AASHTO 2010): 1. For a given crash type and land use, identify the alternative model for which a suitably large dataset can be assembled with information for each of the variables included in the model. Note that models with more variables are preferred. The calibration sample should average at least 100 crashes per year for the most recent 3 years and contain at least 10 corridors. If the data cannot be assembled for all crash and land use types, then the procedure is based on datasets that are available, and the resulting calibration factor estimated in Step 3 is applied to the other crash type and land use type models for which calibration data are unavailable. This assumes the calibration factor is consistent among crash types and land use, which may or may not be appropriate for a given jurisdiction. For example, if the more serious injury crashes are consistently reported and rear-end crashes (typically less serious) are underreported in a jurisdiction, then it would not be appropriate to assume the calibration factor is the same for these two crash types. 2. Use the model for the region deemed most similar to the jurisdiction of interest to estimate the sum of predicted crashes over all corridors in the dataset for the 3-year period. Note that assessing which region is most similar to the calibration dataset is not critical for this purpose; the region is mainly used to define a base condition. However, experience-based judgment may be used in assessing the reasonableness of the calibration factor estimated in Step 3. 3. Estimate the calibration factor as the ratio of the sum of the observed crashes in the calibration dataset to the sum of the predicted crashes from the model output. 4. Apply the calibration factor to the base region multiplier to obtain the multiplier for the jurisdiction of interest. Example: Suppose it is desired to calibrate the model for predicting total crashes on commer- cial corridors in a jurisdiction. A total of 10 commercial corridors are identified for use in the calibration process. The dataset includes 328 reported crashes for the 3-year period. Note there are more than 100 crashes per year for the 10 corridors combined, so this satisfies the require- ments in Step 1 of the calibration process. Step 1. Based on a review of Table 70, there are two alternative models for predicting total crashes on commercial corridors. Model 1 includes access density (ACCDENS) and signal density (SIGDENS). Model 2 includes proportion of corridor with no development (PROPNODEV). A P P E N D I X C Calibration

Calibration 139   If data are available for ACCDENS and SIGDENS for each of the 10 corridors, then Model 1 would be selected because it includes more variables than Model 2. In this example, assume that data are not available for ACCDENS, but data are available for PROPNODEV. Thus, Model 2 is selected for use in calibration based on availability of data. The detailed information for Model 2 is provided in Table E-15, and the model form is given by the equation in Figure C-1, where exp(intercept+region) is the regional multiplier. Figure C-1. Crash prediction model with regional calibration. Step 2. Based on a comparison of local roadway characteristics and crash statistics, it was determined that the data from the corridor of interest is most similar to the sample of data from Minnesota. The model is applied to predict crashes for each of the 10 corridors in each year, and the results are summed. A total of 360.25 crashes is predicted for the 10 corridors over the 3-year period. Step 3. The estimated calibration factor is calculated using the equation in Figure C-2. Figure C-2. Estimated calibration factor. Step 4. The original multiplier from Table E-15 is shown in Figure C-3. Figure C-3. Original multiplier. The calibrated multiplier for use in the jurisdiction of interest is shown in Figure C-4. Figure C-4. Estimation of calibrated multiplier. The calibrated model for the jurisdiction is shown in Figure C-5. Figure C-5. Minnesota crash prediction model. In the same way, multipliers can be obtained for all other crash types and land use types of interest. If data are unavailable or insufficient to estimate a calibration factor for specific crash types for the same land use type (commercial in this example), the calibration factor for total crashes (0.910) can be applied to the models for other crash types of interest for commercial corridors. Similarly, if data are unavailable or insufficient to estimate a calibration factor for other land use categories for the same crash type (total crashes in this example), the calibration factor for total crashes (0.910) can be applied to the models for total crashes for other land use categories of interest.

Next: Appendix D - Summary Statistics by Land Use and Region »
Application of Crash Modification Factors for Access Management, Volume 1: Practitioner's Guide Get This Book
×
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

While research and empirical evidence have shown positive safety and operational benefits associated with good access management practices, it can be challenging for transportation agencies to implement access management strategies on the basis of safety performance without methods and tools to quantify the safety performance of alternatives.

The TRB National Cooperative Highway Research Program's NCHRP Research Report 974: Application of Crash Modification Factors for Access Management, Volume 1: Practitioner’s Guide presents methods to help transportation planners, designers, and traffic engineers quantify the safety impacts of access management strategies and make more informed access-related decisions on urban and suburban arterials.

NCHRP Research Report 974: Application of Crash Modification Factors for Access Management, Volume 2: Research Overview documents the research process related to access management features.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  6. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  7. ×

    View our suggested citation for this chapter.

    « Back Next »
  8. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!