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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations

 
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Publication Number:  FHWA-HRT-12-048    Date:  November 2013
Publication Number: FHWA-HRT-12-048
Date: November 2013

 

Pavement Marking Demonstration Projects: State of Alaska and State of Tennessee

APPENDIX D. SAFETY ANALYSES ON MULTILANE HIGHWAYS

The Illinois freeway crash data were compiled from 2001-2006 from 571 segments (708.1 mi). Table 93 provides a summary of crash rates for those 571 freeway segments. Crashes coded as "intersection" were removed from all crash counts.

Table 93. Summary of Illinois 2001-2006 freeway crash data.


Variable

4-Inch Edge Lines
4-Inch Skip Lines

4-Inch Edge Lines
6-Inch Skip Lines

5-Inch Edge Lines
5-Inch Skip Lines

Total segment length

593.1

21.4

93.6

Number of years

6

6

6

Total number of accidents

31,189

432

1,751

Crash Type

Crash Rates (per million vehicle miles)

Total

0.84

0.50

0.53

F+I

0.17

0.12

0.10

PDO

0.67

0.39

0.43

Daytime

0.47

0.21

0.23

Nighttime

0.33

0.26

0.26

Daytime F+I

0.10

0.07

0.06

Nighttime F+I

0.06

0.04

0.04

Wet

0.10

0.04

0.05

Wet nighttime

0.04

0.02

0.03

Single-vehicle

0.38

0.39

0.42

Single-vehicle wet

0.06

0.04

0.04

Single-vehicle nighttime

0.20

0.22

0.24

Older driver (≥ 55 years old)

0.16

0.09

0.11

Opposite direction

0.01

0.00

0.00

Sideswipe same direction

0.14

0.04

0.05

Fixed object

0.20

0.13

0.10

The Michigan freeway crash data were compiled from 2001-2007 from 508 segments (1,067.4 mi). The annual aggregated crash counts from those 508 freeway segments are provided in table 94. Crashes coded as "intersection" or "interchange" have been removed from all crash counts.

Table 94. Annual aggregated crash counts over 508 freeway segments (1,067.4 mi)
in Michigan for 2001-2007.


Crash Type

Year

2001

2002

2003

2004

2005

2006

2007

Total

1,364

1,347

1,466

1,692

1,556

1,529

1,489

F+I

294

288

288

313

301

291

272

PDO

1,070

1,059

1,178

1,379

1,255

1,238

1,217

Daytime

856

854

879

1,058

963

900

881

Nighttime

400

387

448

519

480

498

484

Daytime F+I

207

188

187

219

217

200

198

Nighttime F+I

68

82

82

77

65

72

60

Wet

202

200

176

217

137

245

163

Wet nighttime

56

44

53

73

37

64

48

Wet F+I

50

38

29

49

30

62

34

Single-vehicle

774

696

815

993

897

948

977

Single-vehicle wet

112

98

113

141

83

158

113

Single-vehicle nighttime

327

280

357

423

399

416

424

Single-vehicle F+I

163

133

149

162

164

146

151

Single-vehicle nighttime F+I

53

51

53

51

41

48

40

Single-vehicle wet F+I

31

17

17

31

17

38

22

Note: Crashes in 2004 are not included in the safety analysis because 2004 is the year of wider line installation.

The research team performed a safety evaluation of Michigan freeway crash data using an interrupted time series approach that introduces time as a variable to control for baseline trend and intervention (installation of wider lines) as a variable to estimate the effect of the wider lines. Because the time series plots of freeway crashes (aggregated crashes over all segments) by year indicated a possible change in the trend (not just in the level) in crashes after the intervention, a new variable, "time after intervention," coded as "0" before the intervention and (time - t0), where t0 is the year of the intervention, was also included to estimate the change in the trend in the expected number of crashes. In addition to time, intervention, and time after intervention, lane width, terrain, log(AADT), log(segment length), and log(number of rainy days) were included as predictors in the negative binomial regression model. GEEs were used as an estimation method to account for correlations in crash counts from multiple years over segments. Table 95 contains the results of applying interrupted time series approaches to the Michigan non-intersection/interchange non-winter month crash data from 508 segments (1,067.4 mi) of freeways with 3 years (2001-2003) of before and 3 years (2005-2007) of after data. It can be observed from the table that there was no statistically significant change in the level of or trend in the expected number of freeway crashes after the installation of wider lines (p-values for intervention or time after intervention were all greater than 0.05), regardless of the crash types considered in the table. That is, no consistent or statistically significant safety effects of wider lines were observed for the Michigan freeway crash data.

Table 95. Results of interrupted time series approaches applied to the Michigan non-intersection/interchange
non-winter month crash data.


Variable

Intercept

Time

Intervention

Time After Intervention

Lane Width

Terrain

Log (AADT)

Log (Segment Length)

Log (Number of Rainy Days)

Total crashes

-6.7012

-0.0089

0.0831

0.0208

0.0064

-0.0806

0.5997

1.3521

0.1682

F+I

-15.9347

-0.0715

0.1936

0.0524

0.3270

0.2609

0.9636

1.4891

0.0831

PDO

-6.0226

0.0117

0.0640

-0.0002

-0.0593

-0.0728

0.5784

1.3357

0.1973

Daytime

-14.4469

-0.0252

0.1077

0.0339

0.3934

-0.0555

0.8498

1.3990

0.1700

Nighttime

-0.0588

0.0121

0.1723

-0.0182

-0.3580

-0.0798

0.3279

1.3128

0.0140

Daytime F+I

-15.8048

-0.1267

0.3517

0.1211

0.1888

0.4336

1.0686

1.5658

0.0220

Nighttime F+I

-19.2711

0.0725

-0.2293

-0.1077

0.5150

0.3201

0.9362

1.4485

0.1507

Wet

-15.8378

-0.0059

-0.1984

0.1047

0.0310

-0.3612

0.7493

1.2577

1.3567

Wet nighttime

-14.7010

0.0840

-0.3696

0.0035

0.0785

-0.5859

0.4336

1.0224

1.5177

Wet F+I

-26.6570

-0.2133

0.4955

0.2534

0.4938

0.5057

0.9269

1.2374

1.4791

Single-vehicle

-0.8200

-0.0041

0.0971

0.0567

-0.1900

-0.1669

0.2324

1.3268

0.0815

Single-vehicle wet

-8.3744

0.0735

-0.3716

0.0903

-0.2281

-0.4371

0.4063

1.1960

1.1056

Single-vehicle nighttime

0.7417

0.0205

0.1645

-0.0008

-0.3031

-0.1165

0.1442

1.2660

0.0452

Single-vehicle V+I

-10.7619

-0.0801

0.2421

0.0511

0.2354

0.2252

0.5641

1.4455

-0.0618

Single-vehicle nighttime F+I

-16.4857

0.0040

-0.1630

-0.0186

0.5384

0.0848

0.5376

1.3888

0.2870

Single-vehicle wet F+I

-19.3663

-0.2613

0.5892

0.3502

0.2290

0.6558

0.6542

1.1711

1.0341

Note: GEE approach was used as an estimation method. Bold represents statistically significant results at α = 0.05.

 

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