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Four-Lane to Three-Lane Conversions In Iowa: A Summary

Four-Lane to Three-Lane Conversions In Iowa: A Summary. Results from Several Research Studies of the Impacts on Safety. Iowa Department of Transportation Office of Traffic and Safety Iowa State University Center for Transportation Research and Education. Preface.

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Four-Lane to Three-Lane Conversions In Iowa: A Summary

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  1. Four-Lane to Three-Lane Conversions In Iowa:A Summary Results from Several Research Studies of the Impacts on Safety Iowa Department of Transportation Office of Traffic and Safety Iowa State University Center for Transportation Research and Education

  2. Preface • The research results I am presenting belong to others. • Their permission in doing so is greatly appreciated.

  3. The Researchers • Michael Pawlovich, Iowa DOT • Alicia Carriquiry, ISU Statistics Department • Win Lee, ISU Statistics Department • Tom Stout, CTRE • Keith Knapp, formerly CTRE • Karen Giese, formerly CTRE • Tom Welch, Iowa DOT (sponsor)

  4. Presentation Outline • 4 lane to 3 lane basics • Results from “traditional” safety studies • Results from Bayesian safety study • Conclusions

  5. The Basics • Converting an existing 4-lane roadway to a 3- lane roadway  typically 2-lanes with a center continuous two-way left-turn lane (TWLTL) • The addition of the turn lane is the critical ingredient for improved traffic flow and safety • May free up enough room for bike lanes or parking Source: Huang et al./HSIS 2002,2004,2005

  6. The Iowa 4 Lane To 3 Lane Experience Before • Iowa DOT sees these conversions as positive in terms of both safety and quality of life After

  7. Conversion Guidelines • When conversions seem to make sense • ADT < 20,000 • Urban or small city arterial or collector with a posted speed of 30 to 45 mph • History of left-turn related crashes • Broadside and rear end collisions • Significant commercial and/or residential driveway density • Question: is there a density that is too high? • Significant left-turning volumes

  8. Source: Iowa DOT RoadView Two Example 4-Lanes Clear Lake, IA – US 18 EB (west of Clear Lake) Storm Lake, IA – US 7 EB  !!! Left-turning vehicle slows van

  9. Source: Iowa DOT RoadView Two Example 3-Lanes Storm Lake, IA – US 7 EB Storm Lake, IA – US 7 EB Left-turning vehicle has no effect

  10. Drive Through:Sioux Center, IAUS 75 NB

  11. 1/8 Multiple signs Long transition length Source: Iowa DOT RoadView

  12. 2/8 From 45 mph Source: Iowa DOT RoadView

  13. 3/8 Passed after signage Source: Iowa DOT RoadView

  14. 4/8 Plenty of time to change lanes Source: Iowa DOT RoadView

  15. 5/8 Left-turning van 4-lane to 3-lane transition Opposing flow queue headways Source: Iowa DOT RoadView

  16. 6/8 Multiple left-turning pickups Parallel Parking Opposing flow queue headways Source: Iowa DOT RoadView

  17. 7/8 Signing and traffic markings Source: Iowa DOT RoadView

  18. 8/8 3-lane to 4-lane transition Source: Iowa DOT RoadView

  19. Drive Through:Sioux Center, IAUS 75 SB

  20. 1/8 From 45 mph to 35 mph Multiple signs Long transition length Source: Iowa DOT RoadView

  21. 2/8 Multiple signs Source: Iowa DOT RoadView

  22. 3/8 3-lane section begins Source: Iowa DOT RoadView

  23. 4/8 Parallel Parking Multiple left-turning vans, pickup to turn right-on-red soon Source: Iowa DOT RoadView

  24. 5/8 Parallel Parking Van finishes left movement, pickup turns right-on-red Source: Iowa DOT RoadView

  25. 6/8 $1.52/gallon!!! Parallel Parking Right-turning car, van waiting for opposing left, opposing thru traffic not hindered Source: Iowa DOT RoadView

  26. 7/8 Pedestrian signs Parallel Parking More lefts and thru traffic Source: Iowa DOT RoadView

  27. 8/8 3-lane to 4-lane transition Source: Iowa DOT RoadView

  28. Analysis History • Treatments in Iowa (All Potential Case Studies) • 15 from 1993 – 2003 • 2001: Guidelines (CTRE) • 4 to 3 Conversion Safety Impact Studies • 2002: Huang et al. (FHWA/HSIS) • Showed small reduction in crash density and no reduction in crash rate (this study is the “odd duck” here) • Fall 2004: Matched-Pair Before/After (CTRE) • Spring 2005: Bayesian Before/After (ISU Statistics Department)

  29. Yoked Pair Comparison • 4-3 conversion segments were compared with similar “control” segments • City population within 20% • Corridor traffic (AADT) within 20% • Physically similar (length, land use, access management, etc.)

  30. Yoked Pair Results • Net effect – 27 to 28% reduction in crash rate

  31. Crash Rate Analysis • Before – 8.86 per MVM • After – 4.45 per MVM

  32. Older Driver Analysis • How did older drivers fare? • Age 65 and older – average crash rate of 7 before (on the 4 lane), 2.9 after (on the 3 lane) • Age 75 and older – average of 3.3 before, 1.5 after

  33. Younger Driver Analysis • How about younger drivers? • Age 25 and younger – average of 28.1 before, 13.3 after • If you have teenagers, now you know why their insurance rates are so high!

  34. Injury Crashes Only • Net benefit, 27% reduction in injury crashes

  35. Conclusions • Overall crash frequency reduction of 21% • Improved crash rate (27-28% reduction) • Improvements for both older and younger drivers • Reduced injury numbers and rate

  36. Bayesian Before/After Study • Objective: Assess whether 4 lane to 3 lane conversions appear to result in crash reductions on Iowa roads • Control for “regression to the mean” phenomenon • Sometimes, crash rate improvements are simply random • Much more rigorous study than the “traditional” study just described

  37. Bayesian Before/After • Data • 15 treatment and 15* control • 23 years (1982-2004) • Volumes: 2,030 to 15,350 (1982-2004) * One control site served as a matched pair for two treatment sites.

  38. Data  3-lane (treatment sites)

  39. Data  4-lane (control sites)

  40. Data  Pairings* Treatment Control * Not essential for Bayesian, more for discussion purposes

  41. Monthly Crash Densities Treatment Control

  42. Monthly Crash Rates Intervention

  43. Modelling/Statistics • Crashes: random, rare count events • Model: Poisson w/ log mean expressed as a piece- wise linear function of: - time period (months) - seasonal effects - random effect for each site • Adopted a Bayesian statistical framework for estimation of model parameters

  44. Results/Discussion • 25.2% (23.2% - 27.8%) in crashes/mile 25% • 18.8% (17.9% - 20.0%) in crash rate 19%

  45. Results/Discussion • Agrees with: • CTRE Matched-pair Before/After study • Why? Prior Sioux Center speed study • 4-5 mph in 85th percentile free flow speed • 12-14% in percentage of vehicles traveling more than 5 mph over the speed limit (i.e., vehicles traveling 35 mph or higher).

  46. Conclusions • 4 lane to 3 lane conversions appear to be successful treatments for calming traffic under the right conditions • They can be very effective in improving traffic safety • They are not especially problematic for either elderly or young drivers

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