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Topic 2 – Network Screening CEE 763

Topic 2 – Network Screening CEE 763. OBJECTIVES. Identify locations for further study which have both A high risk of crash losses An economically justifiable opportunity for reducing the risk Identify countermeasure options and priorities which maximize the economic benefits

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Topic 2 – Network Screening CEE 763

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  1. Topic 2 – Network Screening CEE 763

  2. OBJECTIVES • Identify locations for further study which have both • A high risk of crash losses • An economically justifiable opportunity for reducing the risk • Identify countermeasure options and priorities which maximize the economic benefits • It is as much about exclusion of sites from consideration as it is about inclusion

  3. NETWORK SCREENING • Key tool in a highway safety improvement program • Definition– • A process which aims to identify locations within the road system where correctable crashes are found in order to develop appropriate and cost-effective treatments to reduce the frequency or severity of crashes

  4. EFFECTIVENESS • It is important to identify sites with the most “promise” for improvement as engineering studies are expensive. Agencies have limited budgets, and if a site with potential is not identified, an opportunity to substantially improve safety is missed.

  5. SOME TYPICAL NAMES • High crash location • High accident potential • Black spot • High risk location • Top 5% • Crash concentration

  6. Terms: Site and Facility • Site – a basic safety study location, e.g., a segment (homogeneous), an intersection, and a freeway ramp • Facility – a contiguous set of sites • Freeway (segments, ramps) • Urban and suburban arterials (segments, intersections): divided, undivided, signalized, TWSC etc. • Rural highway (segments): two-lane, multi-lane • HSM only covers predictive methods for certain facility types

  7. NETWORK SCREENING PROCESS • Establish focus • Sites with potential to reduce crash frequency • Specific crash types or severity • Identify sites and reference population • Type of site: segments, intersections, ramps • Sites of similar characteristics • Select performance measures • Frequency, rate, severity, etc. • Select screening method • Ranking, sliding window, peak searching etc. • Screen and evaluate results

  8. ESTABLISH FOCUS

  9. PERFORMANCE MEASURES • Crash frequency* • Crash rate* • Quality control* • Excess predicted crash frequency using method of moments • Critical rate • Crash severity* • Equivalent property damage only (EPDO) crash frequency • Relative severity index • Level of service of safety • Excess predicted average crash frequency using SPFs* • Probability of specific crash types exceeding threshold proportion • Excess proportion of specific crash types • Expected crash frequency with EB adjustment* • Excess expected crash frequency with EB adjustment

  10. CRASH FREQUENCY • Method • Rank locations with highest count of crashes for investigation • Benefits • Simple • Focuses on areas with most crashes • limitations • Does not account for exposure • Favors high-volume, urban locations • Engineering fix may not be present

  11. CRASH RATE • Method • Rank locations by rate of crashes • Benefits • Accounts for exposure • Relatively simply • Efforts focused on potential problem not just high volume locations • Limitations • Favors low volume, low collision sites • Cannot compare cross different volumes

  12. INTERSECTION RATES • Crashes per million entering vehicles (MEV) • Ri = intersection crash rate • N = number of crashes in the study period • n = number of years in the study period • TEV = the sum of volumes entering from all approaches, in Average Daily Traffic

  13. EXAMPLE Observed 46 crashes in two years. The ADT for the minor approach was 3000 and the major approach was 6000. Note - volumes includes both directions. What is the crash rate?

  14. SEGMENT RATES • Crashes per million vehicle miles of travel (MVMT) • Example • Observed 40 crashes on a 17.5 mile segment in one year. The ADT was 5,000.

  15. CRASH AND VOLUME

  16. FREQUENCY-RATE CRITERIA • Method • Rank by combination of frequency and rate based methods • Various ways to combine rankings for composite rankings • Benefits • Simple • Address drawbacks of both the frequency and rate methods • Drawbacks • Final ranking dependent of combination

  17. EXAMPLE • Five intersections have the following crash frequency and crash rate. If a critical frequency is set at 10, and a critical crash rate is set at 1.5, which intersection(s) should be ranked as high crash locations?

  18. QUALITY CONTROLRate or Frequency • Method • Rank location if the crash rate or frequency at a site is statistically significantly higher than a predetermined rate or frequency for locations of similar characteristics • Benefits • Based on Poisson distribution • Seems to identify locations with possible treatments • Drawbacks • More data is required • Categorization is key

  19. QUALITY CONTROL • Method • 1) Select average rate or frequency for similar facility • 2) Calculate the critical rate or frequency • 3) Compare actual rate or frequency • 4) Flag or rank if exceeds • RC = critical rate or critical frequency • Ra = the average rate or frequency for similar facility • P = probability constant based on desired level of significance (1.645 for 95%) • M = millions of VMT or entering vehicles

  20. EXAMPLE • There were 40 observed crashes on a 17.5 mile segment in one year. The ADT was 5,000. Given the average rate for similar segments is 1.02 MVMT, does the subject segment exceed the critical rate at 95% confidence?

  21. SEVERITY • Method • Rank locations by weighting the severity of crashes • Benefits • Adds severity to the frequency method • Usually relates to benefit/cost selection • Drawbacks • Dependent on weighting, may concentrate on fatal collisions • Weights are essentially arbitrary since it assigned from global crash costs

  22. EQUIVALENT PROPERTY DAMAGE ONLY (EPDO) CRASH FREQUENCY EPDO = Equivalent property damage only crashes fi = weight for crash type I Ni = number of crashes of type i

  23. EXAMPLE • A location has experienced 2 fatal, 12 injury A, 30 injury B, 40 injury C, and 140 PDO crashes in 5 years. What is the EPDO crashes? • Fatal = $3,400,000 • A = $260,000 • B = $56,000 • C = $27,000 • PDO = $4,000

  24. RELATIVE SEVERITY INDEX (RSI) = relative severity index cost for intersection i RSIj = relative severity index cost for crash type j

  25. RSI EXAMPLE • An intersection has the following crashes. Determine the RSI for this intersection

  26. SAFETY INDICES • Method • Rank locations by creating an index which includes a number of factors such as rates, frequencies, severities, and possibly site data. A weighted average or scores are then combined to calculate a composite index. The “Relative Severity Index” discussed earlier is one of these types. • Benefits • Simple and attempts to combine criteria • Drawbacks • Rank is sensitive to weights of scores which are usually assigned “arbitrarily”

  27. *ODOT SAFETY PRIORITY INDEX SYSTEM(SPIS) • Composite score assigned for frequency, severity, and rate • 3 years data, 0.10 mile sections • Maximum index is 100 • 25 points max for frequency • 25 points max rate • 50 points max severity • Total score = Sum of Indicator values (IV) of Frequency, Rate, and Severity

  28. *SAFETY PRIORITY INDEX SYSTEM Note: Max SPIS score is 100

  29. EXAMPLE • 0 Fatal, 1 A, 0 B, 3 C, 4 PDO. ADT 14,200.

  30. EXAMPLE • 0 Fatal, 1 A, 0 B, 3 C, 4 PDO. ADT 14,200. Answer: SPIS Score = 38.27

  31. POTENTIAL ACCIDENT REDUCTION • Method • Rank or flag locations where the difference between observed and expected crash experience will maximize benefits if their crash history can be reduced to the expected value. • Benefits • Most uses frequency rather than rates • Can account for “regression to the mean” • Drawbacks • Data hungry, expected values must be predicted

  32. EXCESS PREDICTED CRASH FREQUENCY USING METHOD OF MOMENTS • Calculate average crash frequency per reference population • Calculate crash frequency variance • Calculate adjusted observed crash frequency per site • Calculate potential for improvement (PI) per site • Rank site according to PI (highest to lowest)

  33. EXAMPLE • An unsignalized intersection has observed 11 crashes in a year. Suppose among all the unsignalized intersections, the average crashes per year is 8, and the standard deviation of crash for all the intersections is 3. Calculate the PI for this intersection.

  34. EXCESS PREDICTED CRASH FREQUENCY USING SAFETY PERFORMANCE FUNCTIONS • Calculate expected crash frequency using SPF • Calculate excess predicted average crash frequency • Rank site according to the excess frequency

  35. EXAMPLE • An unsignalized intersection has observed 11 crashes in a year. According to the SPF developed for all the unsignalized intersections, the predicted crash frequency per year is 8. What is the excess predicted crash frequency?

  36. K - Observed # of crashes E{k/K} is best estimate for the expected # of crashes SPF E(k) -Modeled # of crashes EMPIRICAL BAYES METHODS Crash Frequence E(k) is the predicted value at similar sites, in crash/year Y is the analysis period in number of years φ is over-dispersion factor Volume

  37. SAMPLE DATA

  38. SAMPLE DATA

  39. CRASH FREQUENCY WITH EB ADJUSTMENT • Step 1 – Calculate the predicted average crash frequency using an SPF • Step 2 – Calculate annual correction factor

  40. CRASH FREQUENCY WITH EB ADJUSTMENT • Step 3 – Calculate EB weighting factor,Note: rely on dispersion factor or variance.

  41. CRASH FREQUENCY WITH EB ADJUSTMENT • Step 4 – Calculate first year EB adjusted average crash frequency.

  42. CRASH FREQUENCY WITH EB ADJUSTMENT • Step 5 – Calculate final year EB adjusted average crash frequency. • Step 6 – Calculate the variance (optional) • Step 7 - Rank sites based on the EB adjusted expected average crash frequency for the final year.

  43. OTHER CRITERIA • Level of service safety (LOSS) • Konokov et al. (Colorado DOT) • Method of moments • PIARC manual • Proportions testing • Exceeding a particular crash type • Rank locations bases on the current annual cost of crashes based on average cost of crash by accident type

  44. WHICH CRITERIA TO USE? • Little consensus on methods • The key issue is how the criteria adopted direct the analyst to consider sites which contributes to the overall road safety goal, namely the maximization of benefits of road safety treatments

  45. METHOD USAGE • All of the methods are in use either alone or in combination • In US states • Crash frequency by 15% • Crash rate or RQC by 15% of agencies • Crash severities by 50% of agencies • Indices by 18% • Other by 16%

  46. MORE PRECISE DEFINATION OF SITE • Three alternatives (Hauer et al., TRR 1784 – Screening the road network for sites with promise) • Based on “Section” • Based on a uniform length of a roadway, e.g., 0.1 mi • Based on a minimum segment that identifies the highest accident frequency while satisfying the statistical limits (i.e., CV).

  47. SEARCHING ALGORITHMS Expected Segment average Segment average does not correspond to the highest Expected Segment average Segments of different length with the highest crash

  48. Roadway Segment 0.0 mi 0.1 mi 0.2 mi 0.3 mi 0.4 mi 0.5 mi 0.6 mi Win # 1 Win # 2 Win # 3 Win # 4 SLIDING WINDOW0.3-mile window with 0.1 increment *The window that has the highest risk is used to rank the segment.

  49. EXAMPLE • A roadway network has ten segments composed of three types of facilities. Using the sliding window method and the crash rate to rank Segments 1 and 2.

  50. More Data • Segment 1 starts at mile post 1.2 and ends at 2.0. Segment 2 starts at mile post 2.0 and ends at 2.4. Segment 1 Segment 2 1.2 2.0 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.1 2.2 2.3 2.4

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