Geni Bahar, P.E. Describe key concepts and applications

Transcription

1 FUNDAMENTALS OF SAFETY Geni Bahar, P.E. Slide 1 Outline Describe key concepts and applications Collision Severity Safety Performance Functions Prediction Models Observed, Predicted and Expected Crash Frequency and Severity Slide 2 Page 1

2 What is Safety? HSM measures safety in terms of crashes Slide 3 Crash Severity Categories in the HSM Most severe injury: K One or more persons died within 30 days of the crash A B C O Most severe injury: Incapacitating injury Most severe injury: Non-incapacitating evident injury Most severe injury: Possible injury No injuries & reportable property damage resulted from the crash Slide 4 Page 2

3 Evolution of Crash Estimation Improvements in statistical methodologies the science of highway safety Slide 5 Methods to Account for Observed Crash Data Attributes Quality and accuracy Reporting thresholds h Differences between jurisdictions Randomness and Change Roadway and exposure variations over time Crash frequency variability, regression-to-themean and selection bias Slide 6 Page 3

4 Natural Variability in Crash Frequency Short Term Average Crash Obse erved Crash Frequency Short Term Average Crash Frequency Short Term Average Crash Expected Average Crash Frequency Years Slide 7 Regression-to-the-Mean (RTM) If we do not account for RTM when selecting sites for improvement Selected sites may not be the most cost effective Some of non-selected sites may really need / benefit from treatment Amount of safety impact due to the treatment is less than estimated since a portion is natural variability Slide 8 Page 4

5 Empirical Bayesian (EB) Method Accounts for RTM Non-linear relationship between crashes & exposure Uses the negative binomial distribution Overcomes the constraints of limited crash data by using predictive relationships (based on similar sites) Slide 9 LET S EXPLORE... GRADUALLY Slide 10 Page 5

6 Safety Performance Function (SPF) Mathematical relationship between crash frequency yp per unit of time (and road length) and traffic volumes (AADT) Used to estimate the predicted average crash frequency for a base condition Product of statistical modeling process Slide 11 Example Safety Performance Functions Road segment SPF: N p (Rural 2-lane) = AADT x L x (365 x 10-6 ) x e Intersection SPF: N p (Rural multi-lane 4-way stop) = e [a + b ln(aadtmaj) + c ln (AADTmin)] where a, b and c vary for intersection type and severity Slide 12 Page 6

7 Example SPF Single vehicle crashes on 3- leg signalized intersections of urban and suburban arterials AADT of minor approach AADT of major approach Slide 13 Part C: Prediction Models by Facility Type & Site Type Chapter 10 Chapter 11 Chapter 12 Rural Two-Lane Rural Multilane Urban and Suburban Two-Way Roads Highways Arterials Undivided Roadway Segments Divided Roadway Segments Intersections Stop Control on Minor Leg(s) 3-Leg 4-Leg Slide 14 Intersections Signalized 3-Leg 4-Leg Page 7

8 Crash Prediction Methodology Crashes/year * N o (Observed frequency) * N e SPF * N p (Expected frequency) (Predicted frequency) AADT Slide 15 Apply Site-Specific Specific EB Method Improves the crash frequency estimate Rd Reduces effects of regression-to-the-mean th Establishes an expected frequency correcting for short term averages SPF and site specific data should be used to modify ALL observed or counts to expected or long-term average Slide 16 Page 8

9 Empirical Bayesian (EB) Method Reliability of a model is function of: Fit of original data How well model was calibrated for local data Use of Predictive models in the HSM Slide 17 Key Concepts in HSM Crash Frequency Number of Crashes and number of years Expected crashes/year Crash Estimation Forecast/ Predict crash frequency Existing roadway for existing conditions during a past or future period Existing roadway for alternative conditions during a past or future period A new roadway for given conditions for a future period Crash Evaluation Effectiveness after implementation Single application of a treatment Group of similar projects for the specific purpose to quantify the effectiveness of a treatment Cost-benefit assessment of projects Slide 18 Page 9

10 In Conclusion: we can now estimate and compare the safety of two facilities Slide 19 THANK YOU QUESTIONS? Slide 20 Page 10