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An Assessment of the National Highway Traffic Safety Administration's Rating System for Rollover Resistance: Special Report 265 (2002)

Chapter: Appendix C: Supplementary Statistical Results

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Page 100
Suggested Citation:"Appendix C: Supplementary Statistical Results." Transportation Research Board. 2002. An Assessment of the National Highway Traffic Safety Administration's Rating System for Rollover Resistance: Special Report 265. Washington, DC: The National Academies Press. doi: 10.17226/10308.
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Appendix C
Supplementary Statistical Results

Tables C-1 through C-7 present the outcomes of the LOGISTIC procedure using the Statistical Analysis System (SAS).1 Listed are:

  • The name of each parameter included in the model;

  • The degrees of freedom (DF) associated with each parameter;

  • The estimated coefficient of the parameter, obtained by maximum-likelihood estimation;

  • The standard error of the coefficient (a measure of precision);

  • The Wald Chi-square statistic, computed as the square of the value obtained by dividing the parameter estimate by its standard error; and

  • The p-value (Pr > ChiSq) for the Wald Chi-square statistic with 1 DF, with a value below 0.05 indicating a significant effect of the associated model parameter if a 5 percent significance level is chosen.

The parameters included in the logistic model are the static stability factor (SSF) and the five “dummy” state variables (i.e., 0,1 variables). Note that Missouri, the sixth state in the data, is omitted in the model; it is the baseline state in the model. For example, using the modeling results shown in Table C-1 and the notation of Equation 7 in Chapter 3, the logit model can be written as follows:

where P is the estimated probability of a rollover given a single-vehicle crash, and the adjustments are as follows:

−0.1910

if STORM = 1 (+0 otherwise)

+0.9276

if FAST = 1 (+0 otherwise)

+0.1279

if HILL = 1 (+0 otherwise)

+0.5224

if CURVE = 1 (+0 otherwise)

−0.0913

if MALE = 1 (+0 otherwise)

+0.3187

if YOUNG = 1 (+0 otherwise)

−0.3664

if OLD = 1 (+0 otherwise)

+0.2578

if DRINK = 1 (+0 otherwise)

1

The LOGISTIC Procedure. SAS Institute Inc. SAS/STAT® User’s Guide, Version 6, Fourth Edition, Volume 2, SAS Institute Inc., Cary, N.C., 1989.
Page 101
Suggested Citation:"Appendix C: Supplementary Statistical Results." Transportation Research Board. 2002. An Assessment of the National Highway Traffic Safety Administration's Rating System for Rollover Resistance: Special Report 265. Washington, DC: The National Academies Press. doi: 10.17226/10308.
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+1.1611

if State = Florida (+0 otherwise)

+0.7852

if State = Maryland (+0 otherwise)

+0.8006

if State = North Carolina (+0 otherwise)

+1.2121

if State = Pennsylvania (+0 otherwise)

+1.4575

if State = Utah (+0 otherwise)

TABLE C-1 Logit Model Results for Data from Six States Combined (See Figure 3-2)

Parameter

DF

Estimate

Standard Error

Wald Chi-Square

Pr > ChiSq

Intercept

1

1.5326

0.0555

762.6947

<.0001

SSF

1

-3.6027

0.0416

7510.7110

<.0001

STORM

1

-0.1910

0.0130

214.9444

<.0001

FAST

1

0.9276

0.0123

5642.4241

<.0001

HILL

1

0.1279

0.0124

106.9493

<.0001

CURVE

1

0.5224

0.0122

1844.5867

<.0001

MALE

1

-0.0913

0.0123

55.2705

<.0001

YOUNG

1

0.3187

0.0119

720.3518

<.0001

OLD

1

-0.3664

0.0405

81.6902

<.0001

DRINK

1

0.2578

0.0157

270.7577

<.0001

dummy_fl

1

1.1611

0.0214

2953.9104

<.0001

dummy_md

1

0.7852

0.0257

932.6290

<.0001

dummy_nc

1

0.8006

0.0192

1742.2279

<.0001

dummy_pa

1

1.2121

0.0200

3686.3054

<.0001

dummy_ut

1

1.4575

0.0296

2417.7396

<.0001

TABLE C-2 Logit Model Results for Data from Six States Combined for Risk Scenario Close to the Minimum (See Figure 3-3)

Parameter

DF

Estimate

Standard Error

Wald Chi-Square

Pr > ChiSq

Intercept

1

1.0804

2.8127

0.1476

0.7009

SSF

1

-3.7387

2.0212

3.4216

0.0643

dummy_fl

1

0.7377

0.7845

0.8842

0.3470

dummy_md

1

0.5256

1.0138

0.2688

0.6042

dummy_nc

1

0.5774

0.8119

0.5058

0.4770

dummy_pa

1

0.4725

0.8263

0.3270

0.5675

dummy_ut

1

1.9178

1.2993

2.1786

0.1399
Page 102
Suggested Citation:"Appendix C: Supplementary Statistical Results." Transportation Research Board. 2002. An Assessment of the National Highway Traffic Safety Administration's Rating System for Rollover Resistance: Special Report 265. Washington, DC: The National Academies Press. doi: 10.17226/10308.
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TABLE C-3 Logit Model Results for Data from Six States Combined for Risk Scenario at the 25th Percentile (See Figure 3-4)

Parameter

DF

Estimate

Standard Error

Wald Chi-Square

Pr > ChiSq

Intercept

1

1.9149

0.3158

36.7562

<.0001

SSF

1

-3.7359

0.2384

245.5733

<.0001

dummy_fl

1

1.1069

0.1341

68.1204

<.0001

dummy_md

1

0.7083

0.1964

13.0090

0.0003

dummy_nc

1

0.7284

0.1494

23.7583

<.0001

dummy_pa

1

1.1200

0.1415

62.6115

<.0001

dummy_ut

1

1.0745

0.2143

25.1399

<.0001

TABLE C-4 Logit Model Results for Data from Six States Combined for Risk Scenario at the Mean (See Figure 3-5)

Parameter

DF

Estimate

Standard Error

Wald Chi-Square

Pr > ChiSq

Intercept

1

3.1380

0.3896

64.8641

<.0001

SSF

1

-4.1671

0.3083

182.6660

<.0001

dummy_fl

1

1.0949

0.1610

46.2240

<.0001

dummy_md

1

0.7980

0.1861

18.3786

<.0001

dummy_nc

1

0.4573

0.1555

8.6466

0.0033

dummy_pa

1

1.0435

0.1521

47.0791

<.0001

dummy_ut

1

1.3395

0.1965

46.4858

<.0001

TABLE C-5 Logit Model Results for Data from Six States Combined for Risk Scenario at the Median (See Figure 3-6)

Parameter

DF

Estimate

Standard Error

Wald Chi-Square

Pr > ChiSq

Intercept

1

2.8052

0.2493

126.5779

<.0001

SSF

1

-3.9525

0.1990

394.5406

<.0001

dummy_fl

1

1.4559

0.0895

264.3365

<.0001

dummy_md

1

0.6796

0.1198

32.1621

<.0001

dummy_nc

1

0.4733

0.0885

28.6180

<.0001

dummy_pa

1

0.9663

0.0978

97.6930

<.0001

dummy_ut

1

1.8160

0.1236

215.9163

<.0001
Page 103
Suggested Citation:"Appendix C: Supplementary Statistical Results." Transportation Research Board. 2002. An Assessment of the National Highway Traffic Safety Administration's Rating System for Rollover Resistance: Special Report 265. Washington, DC: The National Academies Press. doi: 10.17226/10308.
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TABLE C-6 Logit Model Results for Data from Six States Combined for Risk Scenario at the 75th Percentile (See Figure 3-7)

Parameter

DF

Estimate

Standard Error

Wald Chi-Square

Pr > ChiSq

Intercept

1

4.1884

0.4747

77.8564

<.0001

SSF

1

-4.9957

0.3954

159.5922

<.0001

dummy_fl

1

0.7393

0.1901

15.1233

0.0001

dummy_md

1

0.8961

0.2166

17.1126

<.0001

dummy_nc

1

0.3376

0.1571

4.6142

0.0317

dummy_pa

1

1.2998

0.1448

80.5297

<.0001

dummy_ut

1

1.6824

0.2122

62.8738

<.0001

TABLE C-7 Logit Model Results for Data from Six States Combined for Risk Scenario Close to Maximum (See Figure 3-8)

Parameter

DF

Estimate

Standard Error

Wald Chi-Square

Pr > ChiSq

Intercept

1

0.7049

0.5683

1.5388

0.2148

SSF

1

-1.7458

0.4509

14.9929

0.0001

dummy_fl

1

1.5017

0.1989

56.9796

<.0001

dummy_md

1

0.3726

0.5993

0.3865

0.5342

dummy_nc

1

1.6822

0.1713

96.4075

<.0001

dummy_pa

1

0.7427

0.2205

11.3456

0.0008

dummy_ut

1

2.2298

0.4830

21.3100

<.0001
Page 100
Suggested Citation:"Appendix C: Supplementary Statistical Results." Transportation Research Board. 2002. An Assessment of the National Highway Traffic Safety Administration's Rating System for Rollover Resistance: Special Report 265. Washington, DC: The National Academies Press. doi: 10.17226/10308.
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Page 100
Page 101
Suggested Citation:"Appendix C: Supplementary Statistical Results." Transportation Research Board. 2002. An Assessment of the National Highway Traffic Safety Administration's Rating System for Rollover Resistance: Special Report 265. Washington, DC: The National Academies Press. doi: 10.17226/10308.
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Page 101
Page 102
Suggested Citation:"Appendix C: Supplementary Statistical Results." Transportation Research Board. 2002. An Assessment of the National Highway Traffic Safety Administration's Rating System for Rollover Resistance: Special Report 265. Washington, DC: The National Academies Press. doi: 10.17226/10308.
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Page 103
Suggested Citation:"Appendix C: Supplementary Statistical Results." Transportation Research Board. 2002. An Assessment of the National Highway Traffic Safety Administration's Rating System for Rollover Resistance: Special Report 265. Washington, DC: The National Academies Press. doi: 10.17226/10308.
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Next: Appendix D: Rollover Information from NHTSA’s Website »
An Assessment of the National Highway Traffic Safety Administration's Rating System for Rollover Resistance: Special Report 265 Get This Book
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An Assessment of the National Highway Traffic Safety Administration's Rating System for Rollover Resistance: Special Report 265
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TRB Special Report 265 - An Assessment of the National Highway Traffic Safety Administration's Rating System for Rollover Resistance finds that the static stability factor is a useful indicator of a vehicle's propensity to roll over, but that U.S. government ratings for new cars, light trucks, and sport utility vehicles do not adequately reflect differences in rollover resistance shown by available crash data. According to the report, the five-star system should be revised to allow better discrimination among vehicles and incorporate results from road tests that measure vehicle control and handling characteristics.

Following the National Highway Traffic Safety Administration's (NHTSA's) issuance of vehicle ratings to inform consumers about rollover risk, Congress requested a TRB study to evaluate the appropriateness of the rating system. Motor vehicle rollovers involving passenger cars, vans, pickup trucks, and sport utility vehicles result in approximately 10,000 deaths and 27,000 serious injuries each year in the United States. NHTSA developed a five-star rating system to inform consumers about the rollover resistance of passenger cars and light-duty passenger vehicle trucks.

After thoroughly evaluating NHTSA's development of the rating system, the committee that conducted this study concurred with the agency's reliance on a static measure of vehicle stability but pointed out some inadequacies of the statistical model used to relate this static measure to rollover risk. Alternative statistical approaches would provide a better approximation of risk. The rating system itself was found wanting. The procedures used to develop and test the ratings with consumers through focus groups did not provide credible evidence that consumers understood the message about the actual risk associated with a given vehicle. By being limited to only five levels, the system also discarded valuable information. The data developed by NHTSA could be refined to enable consumers to discriminate better among vehicle models with regard to their rollover experience.

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