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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
 (reduction)
- Rate: 34 percent (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
| Arnold’s Park, IA – US 71 NB |
| Arnold’s Park, IA – US 71 NB |
| Arnold’s Park, IA – US 71 NB |
| Harlan, IA |
 |
| School |
 |
| |
| | 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.
|  |
History
- Treatments in Iowa
- 35 from 1981 – 2004
- 28 from 1991 – 2004
- Studies
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 |  |
| Site 4 | | Intervention |
| | Rate |  |
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% (reduction) in crashes/mile
- 34% (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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