DOT HS 811 617 April 2012
Tire-Related Factors in the
Pre-Crash Phase
DISCLAIMER
This publication is distributed by the U.S. Department of Transportation, National Highway
Trafc Safety Administration, in the interest of information exchange. The opinions, ndings,
and conclusions expressed in this publication are those of the authors and not necessarily those
of the Department of Transportation or the National Highway Trafc Safety Administration.
The United States Government assumes no liability for its contents or use thereof. If trade names,
manufacturers’ names, or specic products are mentioned, it is because they are considered essential
to the object of the publication and should not be construed as an endorsement. The United States
Government does not endorse products or manufacturers.
Suggested APA Citation:
Choi, E-H. (2012, April). Tire-Related Factors in the Pre-Crash Phase. (Report No. DOT HS 811
617). Washington, DC: National Highway Trafc Safety Administration.
i
1. Report No.
2. Government Accession No. 3. Recipient's Catalog No.
DOT HS 811 617
4. Title and Subtitle 5. Report Date
A
p
ril 2012
6. Performing Organization Code
Tire-Related Factors in the Pre-Crash Phase
NPO-421
7. Author(s) 8. Performing Organization Report No.
Eun-Ha Choi, Ph.D.
Bowhead Systems Management, Inc.
Contractor working at NCSA
9. Performing Organization Name and Address: 10. Work Unit No. (TRAIS)
Mathematical Analysis Division,
National Center for Statistics and Analysis
11. Contract or Grant No.
National Highway Traffic Safety Administration
1200 New Jersey Avenue SE., Washington, DC 20590
12. Sponsoring Agency Name and Address 13. Type of Report and Period Covered
Mathematical Analysis Division, NHTSA Technical Report
National Center for Statistics and Analysis
National Highway Traffic Safety Administration
14. Sponsoring Agency Code
1200 New Jersey Avenue SE., Washington, DC 20590
15. Supplementary Notes
16. Abstract
This study focuses on tire problems as experienced by light vehicles in the pre-crash phase. Of special interest are
tire problems such as blowouts or flat tires, tire or wheel deficiency, tire or wheel failure, and tire degradation.
The effect of crash factors on a vehicle experiencing such tire problems in the pre-crash phase is examined. Tire
tread depth, tire pressure, driving experience, vehicle familiarity, rollover, aggressive driving acts, rollover,
vehicle body type, and climatic and road conditions are the candidate factors. The National Motor Vehicle Crash
Causation Survey (NMVCCS) data from 2005 to 2007 is used in the statistical analyses. This data, collected at the
crash scene, provides information about what happened immediately prior to the crash.
A descriptive analysis of this data brings out the differences among the assigned categories of variables in terms
of the frequencies of vehicles or tires in each category. The configural frequency analysis confirms the association
that certain factors may have with a vehicle experiencing tire problems in the pre-crash phase. Among other
findings, the analysis discovered that a vehicle is more likely to experience tire problems when one or more tires
are underinflated or the vehicle is running on tires with inadequate tread depth. The emergence of tire problems in
the pre-crash phase is significantly more likely than chance if a driver is less familiar with the vehicle or lacks
driving experience.
17. Key Words 18. Distribution Statement
Tire-related crash vehicles, crash factors, Document is available to the public from the National
associated factors, critical event, critical reason Technical Information Service www.ntis.gov
19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Price
Unclassified Unclassified 29
NHTSA’s National Center for Statistics and Analysis 1200 New Jersey Avenue SE., Washington, DC 20590
ii
TABLE OF CONTENTS
List of Tables ................................................................................................................................. iii
List of Figures ................................................................................................................................ iv
List of Acronyms ............................................................................................................................ v
Executive Summary ....................................................................................................................... vi
1. Introduction ......................................................................................................................... 1
2. The NMVCCS data ............................................................................................................. 1
3. The NMVCCS perspective of a crash and emergence of tire problems ............................. 2
4. Factors associated with tire’s crash role ............................................................................. 3
5. Analysis methodology ........................................................................................................ 4
6. Analysis results ................................................................................................................... 5
6.1 Tire damage prior to first harmful event .................................................................. 5
6.2 Tire pressure ............................................................................................................. 6
6.3 Tire tread depth ........................................................................................................ 8
6.4 Vehicle rollover ..................................................................................................... 10
6.5 Recent experience on this vehicle .......................................................................... 11
6.6 Inexperienced driver ............................................................................................... 12
6.7 Roadway related factors ......................................................................................... 13
6.8 Aggressive driving acts .......................................................................................... 14
6.9 Climatic conditions ................................................................................................. 14
7. Summary and discussion ................................................................................................... 15
8. Appendix ........................................................................................................................... 17
9. References ......................................................................................................................... 19
NHTSA’s National Center for Statistics and Analysis 1200 New Jersey Avenue SE., Washington, DC 20590
iii
LIST OF TABLES
Table 1. Categorization of analysis variables ................................................................................................................ 4
Table 2. Observed and expected frequencies by tire’s crash role and presence of prior tire damage ............................ 6
Table 3. Observed and expected frequencies of tires by tire’s crash role and tire inflation status ................................ 7
Table 4. Observed and expected frequencies of tires by TPMS use status and tire inflation status .............................. 8
Table 5. Observed and expected frequencies of tires by tire’s crash role and tire tread depth status ............................ 9
Table 6. Observed and expected frequencies by tire’s crash role and rollover status .................................................. 10
Table 7. Distribution of tire-related crash vehicles over rollover status for each vehicle body type ........................... 10
Table 8. Observed and expected frequencies by tire’s crash role and recent experience to this vehicle ..................... 12
Table 9. Observed and expected frequencies by tire’s crash role and presence of inexperienced driver .................... 13
Table 10. Observed and expected frequencies by tire’s crash role and presence of roadway-related factors .............. 13
Table 11. Observed and expected frequencies by tire’s crash role and presence of driver aggressive acts ................. 14
Table 12. Observed and expected frequencies by tire’s crash role and climatic condition .......................................... 15
NHTSA’s National Center for Statistics and Analysis 1200 New Jersey Avenue SE., Washington, DC 20590
iv
LIST OF FIGURES
Figure 1. Three perspectives of tire problems emerging in the pre-crash phase ............................................................ 2
Figure 2. Tire-related crash vehicles and vehicle body type .......................................................................................... 5
Figure 3. Percentage of tire-related crash vehicles by presence of prior tire damage .................................................... 6
Figure 4. Percentage of tires of the tire-related crash vehicles in each category of tire inflation status (underinflated,
correctly inflated, and overinflated) ............................................................................................................................... 7
Figure 5. Percentage of tires of tire-related crash vehicles in each category of tire tread depth .................................... 9
Figure 6. Percentages of tire-related crash vehicles for each vehicle body type and rollover status ........................... 10
Figure 7. Percentage of tire-related crash vehicles in each category of recent experience to this vehicle. .................. 11
Figure 8. Percentage of tire-related crash vehicles in each category of inexperienced driver’s presence ................... 12
Figure 9. Percentage of tire-related crash vehicles in each category of roadway-related factors’ presence. ............... 13
Figure 10. Percentage of tire-related crash vehicles in each category of aggressive driving acts’ presence ............... 14
Figure 11. Percentage of tire-related crash vehicles by climatic condition ................................................................. 15
NHTSA’s National Center for Statistics and Analysis 1200 New Jersey Avenue SE., Washington, DC 20590
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LIST OF ACRONYMS
TPMS - tire pressure monitoring systems
NMVCCS - National Motor Vehicle Crash Causation Survey
CFA - configural frequency analysis
SUV - sport utility vehicle
FMVSS - Federal Motor Vehicle Safety Standard
NASS-CDS - National Automotive Sampling System - Crashworthiness Data System
FARS - Fatality Analysis Reporting System
OEM - original equipment manufacturer
NHTSA’s National Center for Statistics and Analysis 1200 New Jersey Avenue SE., Washington, DC 20590
vi
EXECUTIVE SUMMARY
This study focuses on tire problems as experienced by light vehicles in the pre-crash phase. Of special
interest are these tire-related events: blowouts or flat tires, tire or wheel deficiency, tire or wheel failure,
and tire degradation. According to a 2003 NHTSA report, an estimated 414 fatalities, 10,275 non-fatal
injuries, and 78,392 crashes occurred annually due to flat tires or blowouts before tire pressure monitoring
systems (TPMS) were installed in vehicles.
1
As a result of tire-related safety concerns, NHTSA
established two new Federal Motor Vehicle Safety Standards: FMVSS No. 138
2
requires TPMS on all
new light vehicles and FMVSS No. 139
3
updated the performance requirements for passenger car and
light-truck radial tires.
Both of these rules became effective on September 1, 2007. The effects of these
rules are expected to continue to increase with time as market penetration increases. This study uses data
collected through the National Motor Vehicle Crash Causation Survey in 2005 to 2007 to focus on tire
problems experienced by light vehicles in the pre-crash phase. Other factors such as inadequate tread
depth, tire underinflation, or extreme climatic conditions could also amplify the emergence of tire
problems in this phase. Factors that were analyzed to assess the emergence of tire problems in the pre-
crash phase include tire pressure, tread depth, vehicle body type, vehicle rollover, driver’s familiarity with
the vehicle, driving experience, aggressive driving behavior, roadway-related factors, and climatic
conditions.
The NMVCCS recorded the sequence of events occurring in the pre-crash phase, including those events
related to tires. In the survey, tire problems experienced by vehicles in the pre-crash phase are recorded as
associated factors, critical pre-crash events, or critical reasons, denying any implication that these
problems actually caused the crash. This study uses the NMVCCS data in a descriptive analysis to
highlight the differences among categories of the associated factors. Configural frequency analysis is
conducted to study the association of these factors with the emergence of tire problems.
The NMVCCS data is a sample of 5,470 crashes representing 2,188,970 crashes at the national level. In 9
percent of these crashes, one or more vehicles experienced tire problems in the pre-crash phase.
Correspondingly, of the estimated 3,889,770 vehicles involved in the NMVVCS crashes, 5 percent
experienced tire problems in the pre-crash phase. Fifty percent of the tire-related crashes were single-
vehicle crashes while only 31 percent of crashes where tire-related crash factors were not cited were
single-vehicle crashes. Some of the results from this study are listed below.
Of the tires that were underinflated by more than 25 percent of the recommended pressure,
approximately 10 percent were in vehicles that experienced tire problems in the pre-crash phase. In
contrast, among the correctly inflated tires, a much smaller percentage (3.4%) belongs to vehicles that
experienced tire problems. Thus, underinflation is not the only cause of tire problems; however, when
tires are underinflated by 25 percent or more, tires are 3 times as likely to be cited as critical events in
the pre-crash phase.
With at least one or more tires with lower tread depths (between 0 and 4/32″), vehicles experienced tire
problems during crash occurrence significantly more than chance. Of tires with tread depth in the range
0 to 2/32″, about 26 percent were in vehicles that experienced tire problems in the pre-crash phase
while only 8 percent of tires with tread depth in the range 3/32 to 4/32″ were in such vehicles.
The percentage of vehicles experiencing tire problems is significantly higher among vehicles that rolled
over as compared to vehicles that did not roll over for all vehicle body types: passenger cars, pickups,
SUVs, and vans. Of all SUVs experiencing tire problems in the pre-crash phase, 45 percent rolled over.
For the other body types (passenger cars, pickups, and vans), fewer than 25 percent of the vehicles
experiencing tire problems rolled over. Thus, tire problems experienced in the pre-crash phase were
more likely to result in a rollover in SUVs than in other vehicle types.
NHTSA’s National Center for Statistics and Analysis 1200 New Jersey Avenue SE., Washington, DC 20590
vii
When drivers were less familiar with the vehicles they were driving, the vehicles experienced tire
problems in the pre-crash phase significantly more than chance. This was also the case when drivers
were inexperienced and lacked sufficient driver training. Thus, it is likely that inexperienced drivers and
drivers not familiar with the vehicles they are driving pay less attention to tires and tire pressure.
A significantly higher percentage (11.2%) of vehicles were observed to experience tire problems when
one or more roadway-related factors (e.g., wet road, road under water, slick surface) were present in the
pre-crash phase as compared to when no roadway-related factors were cited (3.9%). Thus, the vehicles
running under adverse roadway conditions may become more vulnerable to tire problems.
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NHTSA’s National Center for Statistics and Analysis 1200 New Jersey Avenue SE., Washington, DC 20590
1. INTRODUCTION
In order for a vehicle to handle safely and to use fuel economically, the vehicle’s tires should be in good
condition. Good condition requires regular monitoring and timely maintenance of all tires on, or
associated with, the vehicle. Nevertheless, it is not uncommon to find vehicles on the road, running on
one or more underinflated/overinflated tires or tires with inadequate tread depth. Tire pressure below the
recommended pressure can cause high heat generation that in turn can cause rapid tire wear and blowout.
Similarly, inadequate tread depth can also cause blowouts of tires. Tire-related events such as tire failure
or blowout resulting from tire deficiencies or other factors are risky and often add to the likelihood of
crash occurrence. According to a 2003 NHTSA report, an estimated 414 fatalities, 10,275 non-fatal
injuries, and 78,392 crashes occurred annually due to flat tires or blowouts before tire pressure monitoring
systems were installed in vehicles.
1
When a vehicle starts experiencing tire problems in the pre-crash phase, i.e., immediately prior to the
collision, the time window for attempting a crash avoidance maneuver is extremely small. This makes the
vehicle vulnerable to crash involvement. Also, the risk of collision may be enhanced if one or more crash
factors are present in this phase of the crash. For example, during crash occurrence, tire blowout of a
vehicle running on a wet road or driven by an inexperienced driver may make the crash unavoidable. In
one of the investigated crashes, two rear tires of a crash-involved pickup truck had only 1/32 inch of tread
depth.
4
The driver felt the rear-end of this vehicle “slip” during crash occurrence, probably due to tire
failure. This happened when it was raining and the road was wet. To reduce the number of crashes that
are attributable to tire problems, it is important to study the crash-involved vehicles that experienced tire
problems in the pre-crash phase. The knowledge about the effect of other crash factors on a vehicle
experiencing tire problems in the pre-crash phase can provide a better perspective of the crashes that may
be attributed to tire problems.
The choice of information available for this purpose is limited. The Indiana Tri-Level Study
5
data,
collected in 1979, has tire information but is outdated. Since then, much has changed – the use of radial
tires on vehicles has increased and so has the availability of tire pressure monitoring devices such as
TPMS. Some of the other databases that contain the tire-related information are National Automotive
Sampling System - Crashworthiness Data System
and the Fatality Analysis Reporting System.
Even
though the data pertaining to crash-involved vehicles is updated annually, the information is compiled
much later after the crash has occurred. Thus, these databases provide little clue on what and how tire
problems were experienced by vehicles during crash occurrence. Additionally, both databases lack
information on tire pressure. To obtain the firsthand information about several aspects of crashes,
NMVCCS was conducted by NHTSA’s National Center for Statistics and Analysis in 2005 to 2007. This
includes information about tire-related events such as tire failure and tire blowout that occurred in the pre-
crash phase, as well as other factors present in the crash.
2. THE NMVCCS DATA
During the 3-year period January 2005 to December 2007, NMVCCS collected driver-, vehicle-,
roadway-, and environment-related information from 6,949 crashes. Each of these crashes occurred
between 6 a.m. and midnight and resulted in a harmful event associated with a vehicle in transport.
Additionally, at least one of the first three vehicles in these crashes was a light passenger vehicle towed
due to damage. The aim of NMVCCS was to record an account of the sequence of events that led to the
crash. To achieve this, the crashes were investigated immediately after the crash occurrence without
assigning the fault to the driver, vehicle, or environment.
The NMVCCS data has certain limitations as in any survey. The small sample sizes, due to a large
number of unknowns or data segmentation required for certain types of analyses, may affect the precision
of the estimates. The information in this survey was recorded from driver and witness interviews, vehicle
assessment, and evaluation of the roadway infrastructure. Therefore, caution is needed when interpreting
the results, as some of the variables used in the analysis are subjective in nature. The NMVCCS data also
contains multiple-choice variables whose attributes may define overlapping categories. This may violate
the assumption of mutual exclusiveness required for certain types of analyses.
Of the total 6,949 crashes investigated during July 2005 to December 2007, sampling weights were
assigned to 5,470 crashes to yield a nationally representative sample. The present study analyzes the data
pertaining to these weighted crashes.
3.
THE NMVCCS PERSPECTIVE OF A CRASH AND EMERGENCE OF TIRE PROBLEMS
A crash in NMVCCS is considered as a simplified linear chain
6
of events comprised of “crash-associated
factors,” “movement prior to critical crash envelope,” “critical pre-crash event,” and “critical reason for
the critical pre-crash event” (Figure 1). Among these elements, the critical pre-crash event documents the
circumstances that made the crash imminent. The movement prior to critical crash envelope refers to
movement of the vehicle immediately before the critical pre-crash event. The crash-associated factors
document factors that might have played a role in crash occurrence. The critical reason is the immediate
reason for the critical event and is often the last failure in the causal chain (i.e., closest in time to the
critical pre-crash event).
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NHTSA’s National Center for Statistics and Analysis 1200 New Jersey Avenue SE., Washington, DC 20590
Figure 1. Three perspectives of tire problems emerging in the pre-crash phase
This study considers tire problems in the above perspective, i.e., the emergence of tire problems in the
pre-crash phase as an associated factor, the critical pre-crash event, or the critical reason for the critical
event. However, none of these implies that a particular tire problem caused the crash. In the subsequent
discussion, these tire problems in the pre-crash phase are referred to as “tire-related crash factors” and are
defined as follows.
Tire/wheel deficiency – assessed as an associated factor –
the variable that indicates if the vehicle experienced a tire deficiency/malfunction (e.g., blowout, air out,
etc.) in the pre-crash phase.
Blowout or flat tires – assessed as the critical pre-crash event –
the variable that shows if blow out or flat tire caused loss of vehicle control when in motion.
Tires/wheels failed, other tire degradation – assessed as the critical reason –
the variable that records catastrophic failures (e.g., blowouts, tread separations, wheel separations) and
tire degradation (e.g., bald and/or underinflated tires) that may degrade the vehicle’s handling
characteristics.
Objectcontacted
(Impact)
Associated
factors
Criticalpre‐crash
event
Criticalreasonfor
criticalevent
EmergenceofTireproblems
PRE‐CRASHPHASE
Movement
priorto
criticalcrash
envelope
Criticalcrashenvelope
The focus of this study is on the vehicles that experienced at least one of these tire problems in the pre-
crash phase, namely tire/wheel deficiency, blowout or flat tire, and tires or wheels failed or other tire
degradation.
For the subsequent analy
Active,if
sis and discussion, the com
atleastoneofthe
pos
tire-rela
ite variable “tire’
t
s crash role” is defined as
Tire
'
edcrashfactorsispresentinthepre-crash
scrashrole
In
A crash-involved vehicle for which the “tire’s crash
active,ifnoneofthetire-relatedcra
phasefo
shfa
rthe
c
v
torsisprese
ehicle,
forthevehicle.
ntinthepre-crashphase
role” is active is referred to as a “tire-related crash
vehicle” and a crash in which such a vehicle is involved, a “tire-related crash.” For comparison purposes,
the study also considers vehicles for which the tire’s crash role is inactive. These vehicles are referred to
as “other crash vehicles.”
4. FACTORS ASSOCIATED WITH TIRE’S CRASH ROLE
The tire’s crash role may be associated with other driver-, vehicle-, or environment-related factors. To
study the association of the tire’s crash role with other crash factors, the following variables are
considered.
Vehicle factors
Tire pressure: tire inflation status
Tire tread depth: tire tread depth measured to the nearest 1/32nd of an inch
Tire damage prior to first harmful event: the pre-crash flaws or damage in each tire of the vehicle
(e.g., complete tread separation, partial tread separation)
TPMS: if tire pressure monitoring system is in use or not
Vehicle body type: a vehicle body type describing the general configuration/shape and
distinguishing characteristics of the motor vehicle
Rollover: whether or not the vehicle rolled over
Driver factors
Inexperienced driver: the presence of a driver with a lack of training or driving experience
Recent experience driving this vehicle: the driver familiarity with this vehicle in terms of the
number of times the driver drove this vehicle in the past three months
Aggressive driving acts: the presence of one or more of the aggressive driving acts such as
speeding, tailgating, rapid/frequent lane changes/weaving, accelerating rapidly from stop, and
stopping suddenly.
Environmental factors
Road related factor: the presence of one or more roadway-related factors such as wet roads, road
under water, slick surface, road washed out, potholes, deteriorated road edges, etc.
Month of the crash: month of the year in which crash occurred
For the purpose of analyses, these variables are categorized as shown in Table 1.
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NHTSA’s National Center for Statistics and Analysis 1200 New Jersey Avenue SE., Washington, DC 20590
Cold(Nov.toFeb.),Hot(JulytoSept.),Mild(Mar.toJune,Oct.)
Table 1. Categorization of analysis variables
Variable Categoriesusedinanalysis
Tire’scrashrole Active,Inactive
Lessthan-25,-25to-10,-10to0,0,0to10,10to25,Greater
Impropertirepressure(inpercent)
than25
Tiretreaddepth(in1/32ndofaninch) 0-2,3-4,5-6,7+
Tiredamagepriortofirstharmfulevent Yes,No
TPMSinuse Yes,No
Vehiclebodytype Passengercars,Pickups,SUVs,Vans
Rollover Yes,No
Inexperienceddriver Yes,No
Recentexperiencedrivingthisvehicle
1-10times,Morethan10times
(inlastthreemonths)
Aggressivedrivingact Oneormorepresent,None
Roadwayrelatedfactor Oneormorepresent,None
Climaticcondition
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NHTSA’s National Center for Statistics and Analysis 1200 New Jersey Avenue SE., Washington, DC 20590
5. ANALYSIS METHODOLOGY
Descriptive and configural frequency analyses (CFA) are conducted to study tire problems as experienced
by vehicles in the pre-crash phase. The purpose of the descriptive analysis is to bring out differences
among the vehicle categories as defined by the attributes of the variables, specified in Table 1. This is
done in terms of the percent frequency of vehicles falling in each of these categories. CFA
7, 8
is conducted
to identify other crash factors that are associated with the emergence of tire problems. In certain attributes
of a factor, a vehicle is more likely to experience tire problems as compared to others. For instance, a tire
with inadequate (less than 2/32") tread depth may make the vehicle more vulnerable to experiencing tire
problems as compared to the tire that has adequate tread depth of 5/32" or more. Identifying such profiles
that may have an effect on tire’s crash role will be useful in developing and implementing crash
prevention measures.
CFA and its interpretation:
CFA is a multivariate statistical technique that searches patterns of variables’ categories, which occurred
more often or less often than expected under chance alone. The population units (crash-involved vehicles
or tires, in the present case) are segmented by cross-tabulation of the variables. Each combination of the
variables’ categories used in the segmentation, referred to as a configuration, characterizes a profile of the
population unit (e.g., a vehicle experienced tire problems in the pre-crash phase while being driven on a
wet road). By comparing the observed and expected frequencies for each configuration, CFA explores the
association of tire’s crash role with other factors of interest such as driving experience, tire pressure, tread
depth, aggressive driving acts, climatic conditions, etc. Since the expected frequency in this technique is
estimated based on the assumption of no association between factors, the expected frequency is presumed
to be the outcome of chance alone. Thus, a statistically significant difference between the observed and
expected frequency for a configuration provides evidence of the association a factor has with a vehicle
experiencing tire problems in the pre-crash phase. The Z-statistic is used to confirm if the difference
(positive or negative) is statistically significant. Computational details of the Z-statistic are provided in
the Appendix. All inferences made through CFA bear a 99 percent confidence level. This is the
Bonferroni adjusted level of the initially set 95 percent confidence level. In the subsequent discussion, for
the sake of brevity, the expression “a vehicle experiencing tire problems” is used for the expression “a
vehicle experiencing tire problems in the pre-crash phase.” Such a vehicle is also referred to as a “tire-
related crash vehicle.”
The results of descriptive analysis are presented as bar charts that show percent frequencies of vehicles
falling in different categories of a variable, specified in Table 1. CFA results are presented in tables that
show configurations of variables (defining vehicle profiles) and the corresponding observed and expected
frequencies, as well as the Z-values. The statistical software SAS 9.1.3 is used for these analyses.
9
6. ANALYSIS RESULTS
The 5,470 NMVCCS crashes represent an estimated 2,188,970 crashes at the national level.
Approximately 9 percent (189,917) of the estimated total were “tire-related crashes.” In terms of vehicles,
of the estimated 3,894,507 vehicles involved in the NMVCCS crashes, 197,421 (about 5% of the
estimated total) are “tire-related crash vehicles.” About 50 percent of the tire-related crashes were single-
vehicle crashes while only about 31 percent of other crashes were single-vehicle crashes.
Figure 2(a) shows the percentages of tire-related crash vehicles in each vehicle body type, shown as
shaded portions of bars and of “other crash variables” as un-shaded portions. About 6 percent of
passenger cars, 4.6 percent of SUVs, 4.3 percent of pickups, and 3.5 percent of vans are tire-related crash
vehicles. Figure 2(b) displays the percentage distribution of the tire-related crash vehicles over vehicle
body types. Among the tire-related crash vehicles, the light passenger vehicles account for the largest
percentage (66.3%), followed by SUVs (17.4%), pickups (11.1%), and vans (4.9%).
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NHTSA’s National Center for Statistics and Analysis 1200 New Jersey Avenue SE., Washington, DC 20590
Figure 2(a). Percentage of tire-related crash vehicles
Figure 2(b). Percentage distribution of tire-related
in each vehicle body type
crash vehicles over vehicle body type
Figure 2. Tire-related crash vehicles and vehicle body type
(Data Source: NMVCCS 2005-2007)
15%
10%
5%
0%
5.9%
4.6%
4.3%
3.5%
Passenger
car
Vehicle
SU
body
V
type
Pickup Van
‐tireoffrequency
vehiclescrash
Percent
related
of
100%
90%
70%
80%
66.3%
frequency
60%
vehicles
50%
30%
40%
10%
20%
17.4%
11.1%
Percent
0%
4.9%
Passenger
car
Vehicle
SU
body
V
type
Pickup Van
100%
10%
6.1 TIRE DAMAGE PRIOR TO FIRST HARMFUL EVENT
The variable “tire damage prior to first harmful event” records the flaws or damage in a tire, such as
complete or partial tread separation prior to the first harmful event (i.e., the first event that caused a fatal
or nonfatal injury or property damage). The NMVCCS data shows that of all the vehicles that had prior
tire damage to one or more of their tires, 31.6 percent experienced tire problems and about 68.4 percent
did not experience tire problems (Figure 3). On the other hand, among vehicles with no prior tire damage,
only 4.5 percent were tire-related crash vehicles and 95.5 percent were other crash vehicles. Table 2
presents the results of CFA to detect the association of prior tire damage with tire’s crash role.
Figure 3. Percentage of tire-related crash
vehicles by presence of prior tire damage
(Data Source: NMVCCS 2005-2007)
31.6%
4.5%
0% 10% 20% 30% 40% 50%
Oneormore
None
Percenttire‐related
crashvehicles
Priortiredamge
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NHTSA’s National Center for Statistics and Analysis 1200 New Jersey Avenue SE., Washington, DC 20590
The results show that when there was prior tire
damage, significantly more than expected
vehicles were observed to experience tire
problems under the assumption that tire’s crash
role has no association with prior tire damage
(Z-value = 11.7). The negative Z-value, -1.8
shows that when there was no prior tire damage,
vehicles were observed to experience tire
problems in the pre-crash phase significantly
less than chance.
Table 2. Observed and expected frequencies by tire’s crash role and presence of prior tire damage
Tire’scrashrole Priortire
Observed Expected Z‐value
(Vehicleclassification) damage
Active
Oneormore 31,234 5,173 11.7*
(Tire‐relatedcrashvehicle)
None 152,540 178,601 -1.8*
Inactive
Oneormore 67,603 93,664 -2.6*
(Othercrashvehicle)
None 3,259,734 3,233,673 0.4
*Statistically significant at 95 percent confidence level
(Data Source: NMVCCS 2005-2007)
6.2 TIRE PRESSURE
NMVCCS records both the recommended and measured tire pressures of each tire of the crash-involved
vehicles. These measurements can be used to determine a tire’s inflation status as:
Underinflated, if the recommended tire pressure exceeds the measured pressure
Overinflated, if the measured tire pressure exceeds the recommended pressure
Correctly inflated, if the measured tire pressure is the same as the recommended pressure
In addition to correct tire inflation, three levels of underinflation and overinflation are considered in the
analysis. These include 0 to 10, 10 to 25, and greater than 25 percent underinflation or overinflation
compared to the recommended pressure. TPMS is required to provide a warning to the driver when one or
more tires are 25 percent or more below the recommended pressure on the tire placard.
Figure 4 shows that of all the tires underinflated by more than 25 percent of the recommended pressure,
about 10 percent belonged to tire-related crash vehicles, which is the highest among the three categories
of underinflated tires. About 4 percent of the tires that were underinflated by less than 10 percent and
about 6 percent of those underinflated by 10 to 25 percent belonged to tire-related crash vehicles.
Similarly, of all the tires overinflated by more than 25 percent, about 7 percent were mounted on tire-
related crash vehicles. Figure 4 shows increasing percentages of underinflated or overinflated tires
belonging to tire-related crash vehicles with increasing levels of underinflation or overinflation. Only 3.4
percent of the correctly inflated tires belonged to tire-related crash vehicles. The percentage of other crash
vehicles for each category of tire inflation status can be obtained by subtracting the percentage of tire-
related crash vehicles, presented in Figure 4, from 100. For example, of the correctly inflated tires, 3.4
percent belonged to tire-related crash vehicles and 96.6 percent belonged to other crash vehicles. Note
that these profiles do not include cases that are recorded as “no original equipment manufacturer (OEM)
wheel at this location,” “flat tire” or “unknown.”
7
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NHTSA’s National Center for Statistics and Analysis 1200 New Jersey Avenue SE., Washington, DC 20590
6.7
5.6
5.4
3.4
4.3
6.3
9.6
0246810
25+
10‐25
0‐10
0
0‐10
10‐25
25+
Un derinflation
Overinflation
Correct infla tion
Tireinflation st atus
Percenttires
Figure 4. Percentage of tires of the tire-related crash vehicles in each category of tire inflation status
(underinflated, correctly inflated, and overinflated)
(Data Source: NMVCCS 2005-2007)
The effect of tire inflation on tire’s crash role during the pre-crash phase is studied by conducting CFA.
The results are presented in Table 3. The positive Z-value 5.1 shows that vehicles with tires underinflated
by more than 25 percent of the recommended pressure experienced tire problems significantly more than
expected under the assumption that tire’s crash role has no association with tire inflation. The negative Z-
value -3.3 suggests that with correctly inflated tires, significantly less than expected vehicles were
observed to experience tire problems in the pre-crash phase.
Tire’scrashrole
(Vehicle
classification)
Active
(Tire‐related
crashvehicle)
Inactive
(Othercrashvehicle)
*Statistically significant at 95 percent confidence level
(Data Source: NMVCCS 2005-2007)
Table 3. Observed and expected frequencies of tires by tire’s crash role and tire inflation status
Tireinflationstatus(percent
ofrecommendedpressure)
25+
Underinflation 10‐25 127,183 116,600
81,806 108,334
Correcttirepressure 0 36,439 61,714
0‐10 78,447 82,976
Overinflation
10‐25 75,037 76,942
43,181 37,220
25+ 975,631 1,017,324
Underinflation 10‐25 1,901,357 1,911,939
1,802,934 1,776,407
Correcttirepressure 0 1,037,222 1,011,947
0‐10 1,365,123 1,360,594
Overinflation 10‐25 1,263,560 1,261,655
604,352 610,313
0‐10
25+
0‐10
25+
Observed
103,735
Expected
62,042
Z‐value
5.1*
1.0
-1.7
-3.3*
-0.5
-0.2
1.0
-1.2
-0.2
0.5
0.7
0.1
0.0
-0.2
One of the devices used for monitoring tire inflation is the TPMS. The effect of TPMS use in maintaining
tire pressure is studied using CFA. Note that the category “TPMS not in use” includes vehicles not
equipped with TPMS as well as vehicles equipped with TPMS that was not in use. TPMS was not
required on all newly manufactured light vehicles until September 1, 2007. Therefore, the NMVCCS data
from 2005-2007 was not able to capture a large number of vehicles equipped with TPMS. Only two
percent of the vehicles included in NMVCCS data were equipped with TPMS.
The results are shown in Table 4. The negative Z-value -2.0 shows that significantly less than expected
tires were observed to be extremely underinflated (more than 25%of the recommended pressure) when
TPMS was used, where expected frequencies were obtained under the assumption that TPMS use status
has no association with tire inflation status. The positive Z-value 2.8 suggests that when TPMS was used,
correctly inflated tires were significantly more likely than chance. Two Z-values -6.4 show that
significantly less than expected tires were overinflated by more than 10 percent of the recommended
pressures when this device was used.
8
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NHTSA’s National Center for Statistics and Analysis 1200 New Jersey Avenue SE., Washington, DC 20590
Table 4. Observed and expected frequencies of tires by TPMS use status and tire inflation status
TPMS Extentofunderinflation
Observed Expected Z‐value
inuse oroverinflation
25+ 7,719 11,328
‐2.0*
Underinflation 10‐25 30,846 23,611
1.6
0‐10 29,331 25,538
0.7
Yes
Correcttirepressure 0 43,841 19,040
2.8*
0‐10 34,337 34,024
0.1
Overinflation 10‐25 19,115 37,617
‐6.4*
25+ 6,960 20,991
‐6.4*
25+ 450,026 446,417
0.2
Underinflation 10‐25 923,278 930,513
‐0.3
0‐10 1,002,639 1,006,433
‐0.1
No Correcttirepressure 0 725,572 750,372
‐0.9
0‐10 1,340,574 1,340,887
0.0
Overinflation 10‐25 1,500,967 1,482,466
0.5
25+ 841,291 827,259
0.5
*Statistically significant at 95 percent confidence level
(Data Source: NMVCCS 2005-2007)
FMVSS No. 138 requires TPMS on all light vehicles manufactured after September 1, 2007, that will be
sold in the U.S.
2
Specifically, the TPMS must alert drivers when the inflation pressure in one or more of
their tires falls below 75 percent of the vehicle manufacturer’s recommended cold inflation pressure.
6.3 TIRE TREAD DEPTH
An adequate tire tread depth on all tires of a vehicle is important to maintain proper grip on the road under
different road conditions. NHTSA recommends that tires should be replaced when the tread depth is
2/32”. As a result, FMVSS No. 139 – New pneumatic radial tires for light vehicles, requires treadwear
indicators that enable a person, through visual inspection, to determine if the tire tread depth is at least
one sixteenth of an inch (or 2/32”).
3
NMVCCS records tire tread depth to the nearest 1/32 of an inch
measured on the shallowest grove of the tread. Four ranges of tread depths, namely 0-2/32″, 3- 4/32″, 5-
6/32″, and above 7/32″ are considered in the analysis. The data show that of all the tires observed with
tread depth between 0 and 2/32″, 26.2 percent were mounted on tire-related crash vehicles (Figure 5). In
regard to tires with adequate tire tread depth, the data show that 8 percent of the tires with tread depth in
the range 3-4/32″, 4 percent in the range 5-6/32″, and 2.4 percent above 7/32″ belonged to tire-related
crash vehicles. With tire tread depth in the range 0-2/32″, vehicles were observed to experience tire
problems in the pre-crash phase 3 times more than vehicles with tread depth in the range 3-4/32″. The
percentage of other crash vehicles for each category of tire tread depth can be obtained by subtracting the
percentage of tire-related crash vehicles, presented in Figure 5, from 100. For example, of the tires with
tread depth in the range 3-4/32″, 26.2 percent belonged to tire-related crash vehicles and 73.8 percent
other crash vehicles.
9
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NHTSA’s National Center for Statistics and Analysis 1200 New Jersey Avenue SE., Washington, DC 20590
Figure 5. Percentage of tires of tire-related crash vehicles in each category of tire tread depth
(Data Source: NMVCCS 2005-2007)
CFA wa
s conducted to study association of tread depth with tire’s crash role. The results in Table 5 show
that significantly more than expected vehicles experienced tire problems when they were running on tires
with tread depth between 0 and 4/32" (Z-values 24.8 and 3.9), where expected frequencies were obtained
under the assumption that tire’s crash role has no association with tire tread depth. With tread depth above
4/32", vehicles experiencing tire problems were significantly less likely than chance (Z-values -5.2 and -
10).
Table 5. Observed and expected frequencies of tires by tire’s crash role and tire tread depth status
Tire’scrashrole Tiretreaddepth
Observed Expected Z‐value
(Vehicleclassification) (1/32″)
0‐2 262,869 58,593 24.8*
Active
3‐4 188,763 138,570 3.9*
(Tire‐relatedcrashvehicle)
5‐6 216,085 315,050 -5.2*
7+ 108,080 263,585 -10.0*
0‐2 740,085 944,361 -5.5*
Inactive
3‐4 2,183,177 2,233,370 -0.9
(Othercrashvehicle)
5‐6 5,176,719 5,077,754 1.2*
7+ 4,403,783 4,248,278 2.1*
*Statistically significant at 95 percent confidence level
(Data Source: NMVCCS 2005-2007)
6.4 VEHICLE ROLLOVER
Rollovers are mostly single-vehicle crashes because they usually do not involve a collision with another
vehicle in transport. Some of the possible reasons for a rollover are tire blowout, loss of tire tread, tire
belt peel off, tread separation, and tire bead unseating. These tire-related events can make a vehicle lose
control, especially at high speeds, and eventually rollover.
Table 7.
P
________
NHTSA’s
Figure
(
Data
Percentage of tire-
related crash vehicles
So
6.
6. Percentag
urce: NMVC
1
0%
5%
10%
15%
20%
r
Observed a
Tire’s
(Vehicle
(Tire‐relat
I
(Otherc
*Statisticall
(Data Sour
25%
Table
Distribution of tire-related crash vehicles over rollover status for each vehicle body type
Percentdistributionoftire‐
Estimatednumberof
Vehiclebody Rollover relatedcrashvehiclesby
tire‐relatedcrash
type status rolloverstatusineachvehicle
vehicles
bodytype
Rollover 28,948 22.1%
assengercars
Non‐rollover 101,988 77.9%
Rollover 5,323 24.3%
Pickups
Non‐rollover 16,604 75.7%
Rollover 15,386 44.9%
SUVs
Non‐rollover 18,863 55.1%
Rollover 2,031 21.0%
Vans
Non‐rollover 7,645 79.0%
Rollover 51,687 26.3%
Total
Non‐rollover 145,100 73.7%
(Data Source: NMVCCS 2005-2007)
10
___________________________________________________________________________________________________
National Center for Statistics and Analysis 1200 New Jersey Avenue SE., Washington, DC 20590
es of tire-related crash vehicles for each vehicle body type and rollover status
CS 2005-2007)
5.7%
5.1%
8.4%
3.8%
9.4%
3.4%
8.1%
3.1%
ollover non-rollover rollover non-rollover rollover non-rollover rollover non-rollover
Passengercars Pickups SUVs Vans
nd expected frequencies by tire’s crash role and rollover status
crashrole
Rollover Observed Expected Z‐value
classification)
Active
Yes 51,687 23,130 5.2*
edcrashvehicle)
No 145,733 174,290 -2.1*
nactive
Yes 404,602 433,159 -1.0*
rashvehicle)
No 3,292,484 3,263,926 0.4*
y significant at 95 percent confidence level
ce: NMVCCS 2005-2007)
Figure 6 shows the percentages of tire-related crash vehicles among rollover and non-rollover vehicles by
vehicle body types. Of rolled-over passenger cars, 15.7 percent were tire-related crash vehicles, which is
the highest as compared to other body types: SUVs (9.4%), pickups (8.4%) and vans (8.1%). Among
non-rollover passenger cars, 5.1 percent were tire-related crash vehicles. For all vehicle body types, the
percentage of tire-related crash vehicles among rolled-over vehicles is much higher than the percentage
among non-rollover vehicles. The percentage of other crash vehicles for each vehicle body type can be
obtained by subtracting the percentage of tire-related crash vehicles, presented in Figure 6, from 100. For
example, among rollover passenger cars, 15.7 percent were tire-related crash vehicles and 84.3 percent
were other crash vehicles.
Analysis is conducted to see if rollover is associated with tire’s crash role. CFA results in Table 6 show
that significantly more than expected vehicles experienced tire problems in the event of rollover, as
indicated by the positive Z-value 5.2, where expected frequencies were obtained under the assumption
that tire’s crash role has no association with rollover. Vehicles experiencing tire problems were less likely
than chance if they do not rollover (Z-value -2.1).
When only tire-related crash vehicles are considered (see Table 7), the highest percentage of rollover is
among SUVs (44.9%) as compared to other body types, passenger cars, pickups, and vans (less than 25%).
In contrast, the lowest percentage of non-rollover was among SUVs (55.1%).
6.5
RECENT EXPERIENCE ON THIS VEHICLE
A driver’s ability to handle a vehicle safely depends to a certain extent on his/her familiarity with the
vehicle. NMVCCS recorded this information as driver’s experience on the vehicle he/she was driving.
This is expressed as the number of times the driver drove this vehicle in the past three months. Figure 7
shows that of all the crash-involved vehicles that were driven 1 to 10 times by their drivers, 8 percent
experienced tire problems. This percentage is significantly lower among vehicles that had been driven
more than 10 times by their drivers (5.2%). The percentage of other crash vehicles (92%) among all the
crash-involved vehicles that were driven 1 to 10 times by their drivers can be obtained by subtracting the
percentage of tire-related crash vehicles (8%), presented in Figure 7, from 100. Similarly, the percentage
of other crash vehicles among the crash vehicles that were driven more than 10 times is obtained as 94.8
percent.
Figure
vehicl
this ve
(Data S
_______
NHTSA’s
7. Per
centage of tire-related crash
es in each category of recent experience to
hicle.
ource: NMVCCS 2005-2007)
11
____________________________________________________________________________________________________
National Center for Statistics and Analysis 1200 New Jersey Avenue SE., Washington, DC 20590
CFA analysis is conducted to see if tire’s crash
role is associated with driver’s experience on the
subject vehicle. The Z-value -3.6 in Table 8
suggests that a vehicle driven by a person who is
familiar with the vehicle is significantly less
likely to experience tire problems than expected
under the assumption that tire’s crash role has no
association with driver’s experience on the
subject vehicle.
Table 8. Observed and expected frequencies by tire’s crash role and recent experience to this vehicle
Tire’scrashrole Numberoftimesdrivingthis
Observed Expected Z‐VALUE
(Vehicleclassification) vehicleinthepastthreemonths
1to10times
Active
16,138 14,669 0.3
(Lessfamiliar)
(Tire‐relatedcrash
Morethan10times
vehicle)
139,975 194,371 -3.6*
(Morefamiliar)
1to10 times
Inactive
186,292 187,761 -0.1
(Lessfamiliar)
(Othercrashvehicle)
Morethan10times
2,542,332 2,487,937 1.0*
(Morefamiliar)
*Statistically significant at 95 percent confidence level
(Data Source: NMVCCS 2005-2007)
12
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NHTSA’s National Center for Statistics and Analysis 1200 New Jersey Avenue SE., Washington, DC 20590
6.6
INEXPERIENCED DRIVER
In addition to driver’s experience to the subject vehicle, driving experience in general plays a significant
role in handling a vehicle safely. In addition, an inexperienced driver may not know how to properly
maintain the vehicle, including tires or they may pay less attention to tires or tire pressure. NMVCCS
records a driver as “inexperienced” if the driver lacks training or has less than a year’s driving experience.
Figure 8 shows that of all the vehicles observed with inexperienced drivers, about 12 percent were tire-
related crash vehicles. On the other hand, among vehicles driven by experienced drivers, tire-related crash
vehicles accounted for a significantly smaller percentage (4.8%).
Figure 8. Percentage of tire-related crash
vehicles in each category of inexperienced
driver’s presence
(Data Source: NMVCCS 2005-2007)
The percenta
ge of other crash vehicles for each
category of inexperienced driver’s presence can
be obtained by subtracting the percentage of tire-
related crash vehicles from 100. For example, of
all crash-involved vehicles observed with
inexperienced drivers, about 88 percent were
other crash vehicles while about 12 percent were
tire-related crash vehicles.
CFA results in Table 9 show that significantly
more than expected vehicles experiencing tire
problems during crash occurrence were observed
with inexperienced drivers (Z-value = 3.7),
where expected frequencies were obtained under
the assumption that tire’s crash role has no
association with a driver’s driving experience.
Table 9. Observed and expected frequencies by tire’s crash role and presence of inexperienced driver
Tire’scrashrole Inexperienced
Observed Expected Z‐value
(Vehicleclassification) driver
Yes 28,439 12,981 3.7*
Active
(Tire‐relatedcrashvehicle)
No
133,380 148,838 -1.2*
Yes 211,622 227,080 -0.9*
Inactive
(Othercrashvehicle)
No
2,619,015 2,603,557 0.3
*Statistically significant at 95 percent confidence level
(Data Source: NMVCCS 2005-2007)
13
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NHTSA’s National Center for Statistics and Analysis 1200 New Jersey Avenue SE., Washington, DC 20590
6.7
ROADWAY RELATED FACTORS
The NMVCCS data shows that when road-related factors (wet road, road under water, slick surface, or
road washed out) were present during crash occurrence, 11.2 percent of the crash-involved vehicles were
tire-related crash vehicles (Figure 9). In contrast, in the absence of these factors, only 3.9 percent of
vehicles were as such. The percentage of other crash vehicles for each category of roadway-related
factors’ presence can be obtained by subtracting the percentage of tire-related crash vehicles from 100.
For example, when road-related factors were present, about 89 percent were other crash vehicles while
about 11 percent were tire-related crash vehicles.
Figure 9. Percentage of tire-related crash
vehicles in each category of roadway-related
factors’ presence.
(Data Source: NMVCCS 2005-2007)
CFA results in Table 10 shed light on t
he effect
of adverse road conditions on tire’s crash role.
Z-value 6.9 shows that significantly more than
expected vehicles experienced tire problems
under adverse roadway conditions. When none
of such conditions was present, significantly less
than expected vehicles experienced tire
problems (Z-value -2.6), where the expected
frequencies were obtained under the assumption
that tire’s crash role has no association with
presence of roadway-related factors. The results
of this analysis show that vehicles running under
adverse roadway conditions may become more
vulnerable to tire problems.
Table 10. Observed and expected frequencies by tire’s crash role and presence of roadway-related factors
Tire’scrashrole Roadwayrelated
Observed Expected Z‐value
(Vehicleclassification) factors
Active
Adverse
(Tire‐relatedcrash
vehicle)
None
Adverse
Inactive
(Othercrashvehicle)
None
*Statistically significant at 95 percent confidence l
72,005
124,963
573,661
3,114,310
evel
32,736
164,232
612,930
3,075,041
6.9*
-2.6*
-1.5*
0.6
(Data Source: NMVCCS 2005-2007)
14
___________________________________________________________________________________________________________
NHTSA’s National Center for Statistics and Analysis 1200 New Jersey Avenue SE., Washington, DC 20590
6.8
AGGRESSIVE DRIVING ACTS
Aggressive driving actions such as speeding, rapid or frequent lane changes, weaving, accelerating
rapidly from stop, or stopping suddenly may expedite tire wear or tire failure.
Figure 10. Percentage of tire-related crash
vehicles in each category of aggressive driving
acts’ presence
(Data Source: NMVCCS 2005-2007)
Figure 10 shows that when drivers showed
aggressive driving acts, 6.9 percent of the
vehicles experienced tire problems. In the
absence of such acts, a smaller percentage of
vehicles were observed to experience tire
problems (5.0%). The percentage of other crash
vehicles for each category of aggressive driving
acts’ presence can be obtained by subtracting the
percentage of tire-related crash vehicles from
100. For example, when drivers showed
aggressive driving acts, about 93 percent were
other crash vehicles while 6.9 percent of the
vehicles experienced tire problems in the pre-
crash phase.
In Table 11, there is no significant Z-value, which suggests that the differences between observed and
expected frequencies for all cells are not statistically significant. That is, there is no sufficient evidence to
reject the assumption that tire’s crash role has no association with driver’s aggressive driving behavior.
Table 11. Observed and expected frequencies by tire’s crash role and presence of driver aggressive acts
Tire’scrashrole Presenceof
(Vehicleclassification) driveraggressive
Observed Expected Z‐value
acts
Active
Oneormore
14,890 11,105 1.2
(Tire‐relatedcrash
Inactive
(Othercrashveh
*Statistically significant at
vehicle)
None
Oneormore
icle)
None
95 percent confidence level
173,733 177,518 -0.3
200,600 204,385 -0.2
3,270,876 3,267,091 0.1
(Data Source: NMVCCS 2005-2007)
6.9
CLIMATIC CONDITIONS
Under extreme temperatures, tires are vulnerable to tire degradation, significant loss of tire pressure,
additional flexing, and stress on the sidewalls. These tire conditions may lead to tire failure or even blow
out. In this analysis, three climatic conditions, cold (November to February), hot (July to September), and
mild (March to June, October) are considered based on the month of the year in which a crash occurred.
In the categorization of climatic conditions, temperature variations by State or location are not considered
due to lack of information.
The NMVCCS data shows that 5.5 percent of the vehicles running in hot weather and 5.3 percent in cold
weather experienced tire problems in the pre-crash phase. These percentages are higher than the
percentage (4.6 %) under mild climatic conditions (Figure 11). The percentage of other crash vehicles for
each category of climatic conditions can be obtained by subtracting the percentage of tire-related crash
vehicles from 100. For example, in hot weather 94.5 percent were other crash vehicles while 5.5 percent
were tire-related crash vehicles.
15
___________________________________________________________________________________________________________
NHTSA’s National Center for Statistics and Analysis 1200 New Jersey Avenue SE., Washington, DC 20590
Figure 11. Percentage of tire-related crash
vehicles by
climatic condition
(Data Source: NMVCCS 2005-2007)
CFA results show how the climatic conditions
affect tire’s crash role. In Table 12, there is no
significant Z-value, which suggests that the
differences between observed and expected
frequencies for all cells are not statistically
significant. That is, there is no sufficient
evidence to reject the assumption that tire’s
crash role has no association with climatic
conditions. Note that in this analysis, the
categorization of climatic conditions did not take
into account temperature variations by State or
location.
Table 12. Observed and expected frequencies by tire’s crash role and climatic condition
Tire’scrashrole Climatic
Observed Expected Z‐value
(Vehicleclassification) condition
Active
Cold 66,661 64,045 0.3
(Tire‐relatedcrash
Hot 63,004 58,588 0.5
vehicle)
Mild 67,756 74,788 -0.9
Cold 1,196,746 1,199,362 -0.1
Inactive
Hot 1,092,754 1,097,170 -0.1
(Othercrashvehicle)
Mild 1,407,587 1,400,555 0.2
*Statistically significant at 95 percent confidence level
(Data Source: NMVCCS 2005-2007)
7.
SUMMARY AND DISCUSSION
Tire problems are inherently hazardous to vehicle safety. When these problems emerge in the pre-crash
phase, the time window for attempting a crash avoidance maneuver is normally very small. The
NMVCCS records tire-related issues in the pre-crash phase as associated factors, critical pre-crash events,
and critical reasons for the critical pre-crash event. This study is focused on these perspectives of tire-
related crash vehicles.
About 5 percent of the estimated total number of vehicles involved in the NMVCCS crashes experienced
tire problems during crash occurrence. Passenger cars accounted for about 66 percent of such vehicles.
The findings from CFA highlighted some of the crash factors that have significant association with a
vehicle experiencing tire problems in the pre-crash phase. With prior tire damage, a vehicle experiencing
tire problems is significantly more likely than chance. This is also the case if a vehicle runs on tires
overinflated by more than 25 percent of the recommended tire pressure. A crash vehicle may experience
16
___________________________________________________________________________________________________________
NHTSA’s National Center for Statistics and Analysis 1200 New Jersey Avenue SE., Washington, DC 20590
tire problems during crash occurrence if it runs on tires with inadequate tread depth. Rollover is also
associated with vehicles that experienced tire problems in the pre-crash phase. The vehicles running under
adverse roadway conditions such as wet roads may become more vulnerable to tire problems. Concerning
driver factors, both the lack of driving experience and lack of familiarity with the vehicle are likely to
contribute to a vehicle experiencing tire problems during crash occurrence.
Thus, while tire problems themselves increase the potential of a vehicle’s involvement in crashes, other
crash factors (an inexperienced driver, adverse roadway conditions, etc.) may add to the crash risk due to
their influence on a vehicle experiencing these problems in the pre-crash phase. The findings of this study
emphasize the importance of careful monitoring of tread depth as well as maintaining the proper inflation
pressure of all tires of the vehicle. This monitoring and maintenance can also provide safeguards against
the emergence of tire problems that are likely to appear under adverse road conditions. In addition, less
experienced drivers or people not familiar with the vehicles they are driving should be more cautious to
prevent tire-related crashes.
17
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NHTSA’s National Center for Statistics and Analysis 1200 New Jersey Avenue SE., Washington, DC 20590
8. APPENDIX
Z-statistics based on first order CFA are obtained as follows:
deffN
n
ij
E
ij
E
ij
O
ij
1
Z
,
where
O
ij
is the weighted observed frequency,
E
ij
is the weighted expected frequency, n is the sample
size,
N is the weighted total, and deff is the design effect. Here, deff, the design effect is the ratio of the
variance of a statistic with a complex sample design used in NMVCCS to the variance of that statistic
with a simple random sample. It is computed by using Taylor series expansion.
First order CFA assumes that the variables forming the contingency table under study
(1) may show main effect, and
(2) are totally independent of each other.
The observed weighted freque
ncy for cell (i,…,l) is
,…, ∈ ,…,
,
and is the weigh
∑
where
…
1
u
0
ifaobservationunit
A
maxim m likelihoo
other
d estimator for the expected cell
wise
k
i
s
ince
ll
i,
…
frequency
,
l
t for observation unit k
is
,⋯,
…
...
⋯
…
,
where i
th
of d variables has
categories with i=1,…,
, j=1,…,
, l=1,…,
and N is the weighted
total.
When only two variables (d=2) are considered,
The observed weighted frequency for cell (i,j) is
∑
∈
,
Where
1
The weighted expected fre
0
ifa
other
observa
w
tionunitkisin
quenc
ise
y
is
celli,
j
and
is the weight for observation unit k
.
.
Where is the weighted row sum and is the weighted column sum and N is , the weighted total
sum. More details are provided in Von Eye
8
and Lohr.
10
,
.
.
..
18
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NHTSA’s National Center for Statistics and Analysis 1200 New Jersey Avenue SE., Washington, DC 20590
19
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NHTSA’s National Center for Statistics and Analysis 1200 New Jersey Avenue SE., Washington, DC 20590
9. REFERENCES
[1] NHTSA. (2003, June). “FMVSS No. 139, New Pneumatic Tires for Light Vehicles”, Docket No.
NHTSA-2003-15400-2.
[2] Tire Pressure Monitoring Systems, 49 C.F.R. § 571.138 (2011).
[3] New Pneumatic Radial Tires for Light Vehicles, 49 C.F.R. § 571.139 (2011).
[4] NHTSA. (2008, July). National Motor Vehicle Crash Causation Survey: Report to congress. (Report
No. DOT HS 811 059). Washington, DC: National Highway Traffic Safety Administration.
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