1. Session #4, Forrest Council, Slides

2. The Potential Impacts of a Towaway Reporting Threshold on Driver/User and Roadway Safety Programs Forrest M. Council Senior Research Scientist UNC Highway Safety Research Center

3. Information from Two Studies “Effects of a Towaway Reporting Threshold on Crash Analysis Results,” (Zegeer, et al., TRR 1635, 1998) “A Review Of The Impacts Of The Towaway Reporting Threshold On North Carolina’s Highway Safety Program,” (Lacy, et al., 2001) Both conducted under FHWA’s Highway Safety Information System (HSIS) Project

4. Why HSIS Interest? HSIS is a multi-year, multi-state data system (1985-present) – Crash, roadway inventory, traffic, supplemental files – Annual files from CA, IL, ME, (MI), MN, OH, NC, UT, WA – Data used by FHWA and provided to many other national research projects – www.hsrc.unc.edu/hsis/ or through FHWA site Any change in threshold will affect HSIS data A primary goal of HSIS is to improve safety data

5. The two studies analyze two basic issues – What is the general effect on key crash variables used in driver, vehicle and roadway research if one moves to a towaway threshold? – Are there specific effects on the roadway- related analysis programs conducted by state and local roadway agencies? Identification of “Sites with Promise” (i.e., high- hazard locations) Site-specific analysis of crash patterns to define most appropriate treatment

6. Methods “General Effects” study – Compare crash frequencies for towaway vs. current PDO level in four HSIS states (IL, MN, MI, NC) – Examine “what’s lost” by roadway type, crash type, object struck, vehicle type – Redevelop crash prediction model using only towaway crashes

7. Methods (con’t) “Site-Analysis Effects” study (NC data) – Compare “high crash location” rankings based on PDO and then towaway – Compare crash patterns on collision diagrams – Questionnaire to safety engineers who used both thresholds in developing collision diagrams

8. Results – Percent Crashes “Lost”

9. Percent Crashes “Lost” by Roadway Class

10. PERCENT OF TOWAWAY CRASHES BY ACCIDENT TYPE 0.0% 10.0% 20.0% 30.0% 40.0% 50.0% 60.0% 70.0% 80.0% 90.0% 100.0% Pedestrian/ Bicyclist Run-off-road/ Rollover Run-off-road fixed object Head-on/ Opposite Dir. Angle/Turning Rear-end/ Sideswipe Same Parking/Backing Animal ACCIDENT TYPE PERCENT IL-Towaway MI-Towaway MN-Towaway NC-Towaway

11. PERCENT TOWAWAY BY FIXED OBJECT 0.0% 10.0% 20.0% 30.0% 40.0% 50.0% 60.0% 70.0% 80.0% 90.0% 100.0% Ditch, embankment Streetlight, utility pole Tree Median barrier Guardrail Fence Sign Mailbox FIXED OBJECT PERCENT IL-Towaway MI-Towaway MN-Towaway NC-Towaway

12. PERCENT TOWAWAY BY VEHICLE TYPE 0.0% 10.0% 20.0% 30.0% 40.0% 50.0% 60.0% 70.0% 80.0% 90.0% 100.0% Passenger car Single unit truck Pickup truck Van Truck tractor trailer Motorcycle/ moped Bus Farm equipment VEHICLE TYPE PERCENT IL-Towaway MI-Towaway MN-Towaway NC-Towaway

13. CRASH RATES FOR TOWAWAY VS. TOTAL CRASHES URBAN ROAD CLASSES 1.49 1.77 1.04 4.78 5.75 2.79 1.82 5.77 8.73 4.8 2.71 3.94 5.41 3.56 2.09 0.77 0.87 2.02 2.49 1.57 1.1 2.34 3.66 2.61 1.62 1.69 2.01 1.73 1.34 0.59 0.3 6 0.65 0 1 2 3 4 5 6 7 8 9 ILMIMNNCILMIMNNCILMIMNNCILMIMNNC CRASH RATE (Crashes per 1.61 million vehicle km) TotalTowaway Urban Freeways Urban Divided Multi-lane Urban Undivided Multi-lane Urban Two-lane

14. Results – Model Comparisons Model predicting crashes/mi on rural multilane roads (MN data) – Original Poisson model for total reported crashes – New model for towaway crashes only Not much difference – Predicted frequencies were lower (of course) – Access Control became non-significant predictor – Difference in old and new frequencies much greater for non- principal municipal arterials than for other classes

15. Results – “Site Analyses” Ranking of high crash locations (based on three warrants) – 50% of “top 200” intersections and 50% of “top 200” sections changed – Significant shuffling of ranking within the list (only 4 intersections and 3 sections retained same ranking) – Does it matter? (See later Miller study)

16. Results – “Site Analysis” Collision diagrams – 14 (7 pairs) of diagrams produced and presented in random order – 12 experienced DOT safety engineers circled “definable patterns” and starred those that were “serious and potentially treatable” – (Note that approximately 1/3 of crashes “lost” with towaway threshold)

18. Results – “Site Analysis” (con’t) Collision diagrams – Patterns identified decreased by 35% (247 vs. 160) – Serious patterns decreased by 41% (126 vs. 75) – Crash patterns identified changed “Turning” patterns decreased 43% Rear-end/swipe patterns decreased 67% Thus, countermeasures chosen would both decrease and change – Note that other states may “lose” more crashes and patterns

19. Results – Other Studies Miller, et al., “Sensitivity of a Highway Safety Resource Allocation Model to Variations in Benefit Computation Parameters,” TRR 1124, 1987. – For higher safety improvement budget ($1.2 - $1.5 mill), 20-30 percent of program benefits could be lost by less reporting, among other factors Pfefer and Raub, “Economic Analysis of Highway Safety Data – Final Report,” FHWA, 1995. – Since officer is on the scene for other duties, cost of reporting PDO crash could be as low as $20

20. Conclusions Move to towaway threshold will significantly affect safety data – High loses of some crash types which will affect driver, vehicle and roadway problem identification – Major changes to “HAL” rankings – how much effect? – Significant loss of site-based crash patterns Can’t we improve efficiency by other means, such as new technology? Is the loss of the data worth the relatively low cost of collecting it?