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NCHRP Project 17-35: Evaluation of Safety Strategies at Signalized Intersections

 

 

Evaluations of Low Cost Safety Improvements Pooled Fund Study

 

PPT version for Printing

Iowa’s Experience with Two–lane to Three–lane Conversions

Wen Li, Master’s Student
and
Dr. Alicia Carriquiry
Iowa State University Department of Statistics,

Dr. Michael Pawlovich, PE
and
Tom Welch, PE
Iowa DOT Office of Traffic and Safety

for
the Iowa Department of Transportation
Office of Traffic and Safety

Preliminary Results

  • Density: 12 percent downward arrow (reduction)
  • Rate: 34 percent downward arrow (reduction)

NO control sites – so preliminary results – future effort

What

  • Converting an existing 2–lane roadway to a 3–lane roadway → typically 2–lanes with center continuous left–turn lane
  • Important → left–turn lane
  • Also – sometimes bike lanes and pedestrian accommodations

What: 3–lane

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Arnold’s Park, IA – US 71 NB
 
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Arnold’s Park, IA – US 71 NB
 
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Arnold’s Park, IA – US 71 NB
 
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Indianola, IA – IA 92 NB
 
Harlan, IA
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School
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  Speed Limit

Harlan, IA – US 59 NB

Note: Oncoming van is entering turn lane with vehicle behind it. Speed limit is 45 mph thru the school site.

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History

  • Treatments in Iowa
    • 35 from 1981 – 2004
    • 28 from 1991 – 2004
  • Studies
    • 2005 – Florida DOT

Bayesian Before/After

  • Objective: Assess whether 2–lane to 3–lane conversions appear to result in crash reductions on Iowa roads
  • Data
    • 34 treatment – 1 dropped as treatment in 1981
    • 25 years (1982–2006)
    • Volumes: 2,500 to 15,700 (1982–2006)
    • Currently NO control sites – future effort

Site Data

Site ID City Population 2000 Volume 2000
1 Ames 50,731 6,800
2 Ames 50,731 7,900
3 Ames 50,731 7,500
4 Ames 50,731 8,500
5 Ames 50,731 5,000
6 Ames 50,731 12,200
7 Ames 50,731 10,000
8 Arnolds Park 1,162 15,700
9 Atlantic 7,257 8,000
10 Council Bluffs 58,268 8,200
11 Creston 7,597 5,300
12 Davenport 98,359 9,500
13 Davenport 98,359 8,500
14 Dyersville 4,035 9,000
15 Eldridge 4,159 7,100
16 Harlan 5,282 5,300
17 Hull 1,960 4,400
18 Indianola 12,998 13,500
19 Iowa City 62,220 10,000
20 Iowa City 62,220 7,500
21 Manchester 5,257 7,900
22 Manchester 5,257 7,500
23 Maquoketa 6,112 10,000
24 Mason City 29,172 3,200
25 Mason City 29,172 7,400
26 Nevada 6,658 3,400
27 Nevada 6,658 5,600
28 Nevada 6,658 4,000
29 New London 1,937 2,500
30 Newton 15,579 11,500
31 North Liberty 5,367 11,500
32 Oelwein 6,692 5,000
33 Orange City 5,582 4,500
34 Pella 9,832 3,200
35 Waverly 8,968 6,000

Note: Maquoketa dropped from analysis (1981)

 
Site ID City Length (mi) Completion Year
1 Ames 0.50 2003
2 Ames 0.80 2002
3 Ames 0.35 1998
4 Ames 0.45 1996
5 Ames 0.25 2004
6 Ames 0.30 1998
7 Ames 0.50 2001
8 Arnolds Park 1.80 2000
9 Atlantic 2.00 1994
10 Council Bluffs 0.85 1995
11 Creston 0.32 2004
12 Davenport 1.00 1992
13 Davenport 0.90 2003
14 Dyersville 0.50 1987
15 Eldridge 0.60 2001
16 Harlan 2.17 2001
17 Hull 0.25 1997
18 Indianola 1.60 1983
19 Iowa City 0.45 1997
20 Iowa City 0.75 2001
21 Manchester 0.20 2000
22 Manchester 0.50 2000
23 Maquoketa 0.80 1981
24 Mason City 1.00 2003
25 Mason City 1.00 1998
26 Nevada 0.25 2002
27 Nevada 0.25 2002
28 Nevada 1.00 1999
29 New London 0.25 1985
30 Newton 0.90 1982
31 North Liberty 0.50 2000
32 Oelwein 1.00 1984
33 Orange City 1.00 1991
34 Pella 1.20 1985
35 Waverly 1.85 1997

Note: Maquoketa dropped from analysis (1981)

Monthly Crash Densities and Rates

  Density View alternative text
Site 4   Intervention
  Rate View alternative text

Modeling/Statistics

  • Crashes: random, rare count events
  • Model: Poisson with/ 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
  • Model essentially the same as for prior 4–lane to 3–lane analysis

Preliminary Results/Discussion

  • 12% downward arrow(reduction) in crashes/mile
  • 34% downward arrow(reduction) in crash rate

Recall NO control sites – so preliminary results

Results/Discussion

  • Prior Florida DOT study:
    • 32 percent decline in crashes
    • not "significant" at 95 percent confidence level
  • Need to include control sites – future

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