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Mechanic

Check Tyre Pressures

Tyre-Related Factors in the Pre-Crash Phase


EXECUTIVE SUMMARY
This study focuses on tyre problems as experienced by light vehicles in the pre-crash phase. Of special interest are these tyre -related events: blowouts or flat tyres , tyre or wheel deficiency, tyre or wheel failure, and tyre 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 tyres or blowouts before tyre pressure monitoring systems (TPMS) were installed in vehicles.1 As a result of tyre -related safety concerns, NHTSA established two new Federal Motor Vehicle Safety Standards: FMVSS No. 1382 requires TPMS on all new light vehicles and FMVSS No. 1393 updated the performance requirements for passenger car and light-truck radial tyres . 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 tyre problems experienced by light vehicles in the pre-crash phase. Other factors such as inadequate tread depth, tyre underinflation, or extreme climatic conditions could also amplify the emergence of tyre problems in this phase. Factors that were analyzed to assess the emergence of tyre problems in the precrash phase include tyre 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 tyres . In the survey, tyre 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 tyre 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 tyre problems in the pre-crash phase. Correspondingly, of the estimated 3,889,770 vehicles involved in the NMVVCS crashes, 5 percent experienced tyre problems in the pre-crash phase. Fifty percent of the tyre -related crashes were singlevehicle crashes while only 31 percent of crashes where tyre -related crash factors were not cited were single-vehicle crashes. Some of the results from this study are listed below.

.• Of the tyres that were underinflated by more than 25 percent of the recommended pressure, approximately 10 percent were in vehicles that experienced tyre problems in the pre-crash phase. In contrast, among the correctly inflated tyres , a much smaller percentage (3.4%) belongs to vehicles that experienced tyre problems. Thus, underinflation is not the only cause of tyre problems; however, when tyres are underinflated by 25 percent or more, tyres are 3 times as likely to be cited as critical events in the pre-crash phase.

• With at least one or more tyres with lower tread depths (between 0 and 4/32.), vehicles experienced tyre problems during crash occurrence significantly more than chance. Of tyres with tread depth in the range 0 to 2/32., about 26 percent were in vehicles that experienced tyre problems in the pre-crash phase while only 8 percent of tyres with tread depth in the range 3/32 to 4/32. were in such vehicles.

• The percentage of vehicles experiencing tyre 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 tyre 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 tyre problems rolled over. Thus, tyre problems experienced in the pre-crash phase were more likely to result in a rollover in SUVs than in other vehicle types.

• When drivers were less familiar with the vehicles they were driving, the vehicles experienced tyre 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 tyres and tyre pressure.

• A significantly higher percentage (11.2%) of vehicles were observed to experience tyre 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 tyre problems.

1. INTRODUCTION

In order for a vehicle to handle safely and to use fuel economically, the vehicle’s tyres should be in good condition. Good condition requires regular monitoring and timely maintenance of all tyres on, or associated with, the vehicle. Nevertheless, it is not uncommon to find vehicles on the road, running on one or more underinflated/overinflated tyres or tyres with inadequate tread depth. tyre pressure below the recommended pressure can cause high heat generation that in turn can cause rapid tyre wear and blowout. Similarly, inadequate tread depth can also cause blowouts of tyres . tyre -related events such as tyre failure or blowout resulting from tyre 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 tyres or blowouts before tyre pressure monitoring systems were installed in vehicles.

When a vehicle starts experiencing tyre 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, tyre 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 tyres 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 tyre failure. This happened when it was raining and the road was wet. To reduce the number of crashes that are attributable to tyre problems, it is important to study the crash-involved vehicles that experienced tyre problems in the pre-crash phase. The knowledge about the effect of other crash factors on a vehicle experiencing tyre problems in the pre-crash phase can provide a better perspective of the crashes that may be attributed to tyre problems.

The choice of information available for this purpose is limited. The Indiana Tri-Level Study5 data, collected in 1979, has tyre information but is outdated. Since then, much has changed – the use of radial tyres on vehicles has increased and so has the availability of tyre pressure monitoring devices such as TPMS. Some of the other databases that contain the tyre -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 tyre problems were experienced by vehicles during crash occurrence. Additionally, both databases lack information on tyre 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 tyre -related events such as tyre failure and tyre blowout that occurred in the precrash 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 tyre PROBLEMS

A crash in NMVCCS is considered as a simplified linear chain6 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).




Figure 1. Three perspectives of tyre problems emerging in the pre-crash phase

This study considers tyre problems in the above perspective, i.e., the emergence of tyre 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 tyre problem caused the crash. In the subsequent discussion, these tyre problems in the pre-crash phase are referred to as “tire-related crash factors” and are defined as follows. tyre /wheel deficiency – assessed as an associated factor – the variable that indicates if the vehicle experienced a tyre deficiency/malfunction (e.g., blowout, air out, etc.) in the pre-crash phase.

Blowout or flat tyres – assessed as the critical pre-crash event – the variable that shows if blow out or flat tyre caused loss of vehicle control when in motion. tyres /wheels failed, other tyre degradation – assessed as the critical reason – the variable that records catastrophic failures (e.g., blowouts, tread separations, wheel separations) and tyre degradation (e.g., bald and/or underinflated tyres ) that may degrade the vehicle’s handling characteristics.

The focus of this study is on the vehicles that experienced at least one of these tyre problems in the precrash phase, namely tyre /wheel deficiency, blowout or flat tyre , and tyres or wheels failed or other tyre degradation.

For the subsequent analysis and discussion, the composite variable “tire’ t s crash role” is defined as



A crash-involved vehicle for which the “tire’s crash 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
• tyre pressure: tyre inflation status
• tyre tread depth: tyre tread depth measured to the nearest 1/32nd of an inch .
• tyre damage prior to first harmful event: the pre-crash flaws or damage in each tyre of the vehicle (e.g., complete tread separation, partial tread separation)
• TPMS: if tyre 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.



5. ANALYSIS METHODOLOGY

Descriptive and configural frequency analyses (CFA) are conducted to study tyre 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. CFA7, 8 is conducted to identify other crash factors that are associated with the emergence of tyre problems. In certain attributes of a factor, a vehicle is more likely to experience tyre problems as compared to others. For instance, a tyre with inadequate (less than 2/32") tread depth may make the vehicle more vulnerable to experiencing tyre problems as compared to the tyre 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 tyres , 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 tyre 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, tyre 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 tyre 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 tyre problems” is used for the expression “a vehicle experiencing tyre problems in the pre-crash phase.” Such a vehicle is also referred to as a “tirerelated 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 tyre -related crashes were singlevehicle crashes while only about 31 percent of other crashes were single-vehicle crashes.

Figure 2(a) shows the percentages of tyre -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 tyre -related crash vehicles. Figure 2(b) displays the percentage distribution of the tyre -related crash vehicles over vehicle body types. Among the tyre -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%).



6.1 tyre DAMAGE PRIOR TO FIRST HARMFUL EVENT

The variable “tire damage prior to first harmful event” records the flaws or damage in a tyre , 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 tyre damage to one or more of their tyres , 31.6 percent experienced tyre problems and about 68.4 percent did not experience tyre problems (Figure 3). On the other hand, among vehicles with no prior tyre damage, only 4.5 percent were tyre -related crash vehicles and 95.5 percent were other crash vehicles. Table 2 presents the results of CFA to detect the association of prior tyre damage with tire’s crash role.

The results show that when there was prior tyre damage, significantly more than expected vehicles were observed to experience tyre problems under the assumption that tire’s crash role has no association with prior tyre damage (Z-value = 11.7). The negative Z-value, -1.8 shows that when there was no prior tyre damage, vehicles were observed to experience tyre problems in the pre-crash phase significantly less than chance.




6.2 tyre PRESSURE

NMVCCS records both the recommended and measured tyre pressures of each tyre of the crash-involved vehicles. These measurements can be used to determine a tire’s inflation status as:

• Underinflated, if the recommended tyre pressure exceeds the measured pressure
• Overinflated, if the measured tyre pressure exceeds the recommended pressure~
• Correctly inflated, if the measured tyre pressure is the same as the recommended pressure

In addition to correct tyre 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 tyres are 25 percent or more below the recommended pressure on the tyre placard.

Figure 4 shows that of all the tyres underinflated by more than 25 percent of the recommended pressure, about 10 percent belonged to tyre -related crash vehicles, which is the highest among the three categories of underinflated tyres . About 4 percent of the tyres that were underinflated by less than 10 percent and about 6 percent of those underinflated by 10 to 25 percent belonged to tyre -related crash vehicles. Similarly, of all the tyres overinflated by more than 25 percent, about 7 percent were mounted on tirerelated crash vehicles. Figure 4 shows increasing percentages of underinflated or overinflated tyres belonging to tyre -related crash vehicles with increasing levels of underinflation or overinflation. Only 3.4 percent of the correctly inflated tyres belonged to tyre -related crash vehicles. The percentage of other crash vehicles for each category of tyre inflation status can be obtained by subtracting the percentage of tirerelated crash vehicles, presented in Figure 4, from 100. For example, of the correctly inflated tyres , 3.4 percent belonged to tyre -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.”


Figure 4. Percentage of tyres of the tyre -related crash vehicles in each category of tyre inflation status (underinflated, correctly inflated, and overinflated)
(Data Source: NMVCCS 2005-2007)

The effect of tyre 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 tyres underinflated by more than 25 percent of the recommended pressure experienced tyre problems significantly more than expected under the assumption that tire’s crash role has no association with tyre inflation. The negative Zvalue -3.3 suggests that with correctly inflated tyres , significantly less than expected vehicles were observed to experience tyre problems in the pre-crash phase.

Table 3. Observed and expected frequencies of tyres by tire’s crash role and tyre inflation status



One of the devices used for monitoring tyre inflation is the TPMS. The effect of TPMS use in maintaining tyre 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 tyres 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 tyre inflation status. The positive Z-value 2.8 suggests that when TPMS was used, correctly inflated tyres were significantly more likely than chance. Two Z-values -6.4 show that significantly less than expected tyres were overinflated by more than 10 percent of the recommended pressures when this device was used.

Table 4. Observed and expected frequencies of tyres by TPMS use status and tyre inflation status




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 tyres falls below 75 percent of the vehicle manufacturer’s recommended cold inflation pressure.

6.3 tyre TREAD DEPTH

An adequate tyre tread depth on all tyres of a vehicle is important to maintain proper grip on the road under different road conditions. NHTSA recommends that tyres should be replaced when the tread depth is 2/32”. As a result, FMVSS No. 139 – New pneumatic radial tyres for light vehicles, requires treadwear indicators that enable a person, through visual inspection, to determine if the tyre tread depth is at least one sixteenth of an inch (or 2/32”).3 NMVCCS records tyre 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 tyres observed with tread depth between 0 and 2/32., 26.2 percent were mounted on tyre -related crash vehicles (Figure 5). In regard to tyres with adequate tyre tread depth, the data show that 8 percent of the tyres 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 tyre -related crash vehicles. With tyre tread depth in the range 0-2/32., vehicles were observed to experience tyre 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 tyre tread depth can be obtained by subtracting the percentage of tyre -related crash vehicles, presented in Figure 5, from 100. For example, of the tyres with tread depth in the range 3-4/32., 26.2 percent belonged to tyre -related crash vehicles and 73.8 percent other crash vehicles.



CFA was 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 tyre problems when they were running on tyres 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 tyre tread depth. With tread depth above 4/32", vehicles experiencing tyre problems were significantly less likely than chance (Z-values -5.2 and - 10).

Table 5. Observed and expected frequencies of tyres by tire’s crash role and tyre tread depth status



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 tyre blowout, loss of tyre tread, tyre belt peel off, tread separation, and tyre bead unseating. These tyre -related events can make a vehicle lose control, especially at high speeds, and eventually rollover







Figure 6 shows the percentages of tyre -related crash vehicles among rollover and non-rollover vehicles by vehicle body types. Of rolled-over passenger cars, 15.7 percent were tyre -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 tyre -related crash vehicles. For all vehicle body types, the percentage of tyre -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 tyre -related crash vehicles, presented in Figure 6, from 100. For example, among rollover passenger cars, 15.7 percent were tyre -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 tyre 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 tyre problems were less likely than chance if they do not rollover (Z-value -2.1).

When only tyre -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 tyre 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 tyre -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

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 tyre problems than expected under the assumption that tire’s crash role has no association with driver’s experience on the subject vehicle.





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 tyres or they may pay less attention to tyres or tyre 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 tirerelated crash vehicles. On the other hand, among vehicles driven by experienced drivers, tyre -related crash vehicles accounted for a significantly smaller percentage (4.8%).

The percentage of other crash vehicles for each category of inexperienced driver’s presence can be obtained by subtracting the percentage of tirerelated 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 tyre -related crash vehicles.

CFA results in Table 9 show that significantly more than expected vehicles experiencing tyre 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.




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 tyre -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 tyre -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 tyre -related crash vehicles.

CFA results in Table 10 shed light on the effect of adverse road conditions on tire’s crash role. Z-value 6.9 shows that significantly more than expected vehicles experienced tyre problems under adverse roadway conditions. When none of such conditions was present, significantly less than expected vehicles experienced tyre 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 tyre problems.




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 tyre wear or tyre failure.

Figure 10 shows that when drivers showed aggressive driving acts, 6.9 percent of the vehicles experienced tyre problems. In the absence of such acts, a smaller percentage of vehicles were observed to experience tyre 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 tyre -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 tyre problems in the precrash 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.



6.9 CLIMATIC CONDITIONS

Under extreme temperatures, tyres are vulnerable to tyre degradation, significant loss of tyre pressure, additional flexing, and stress on the sidewalls. These tyre conditions may lead to tyre 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 tyre 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 tyre -related crash vehicles from 100. For example, in hot weather 94.5 percent were other crash vehicles while 5.5 percent were tyre -related crash vehicles.

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





7. SUMMARY AND DISCUSSION

tyre 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 tyre -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 tirerelated crash vehicles.

About 5 percent of the estimated total number of vehicles involved in the NMVCCS crashes experienced tyre 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 tyre problems in the pre-crash phase. With prior tyre damage, a vehicle experiencing tyre problems is significantly more likely than chance. This is also the case if a vehicle runs on tyres overinflated by more than 25 percent of the recommended tyre pressure. A crash vehicle may experience tyre problems during crash occurrence if it runs on tyres with inadequate tread depth. Rollover is also associated with vehicles that experienced tyre problems in the pre-crash phase. The vehicles running under adverse roadway conditions such as wet roads may become more vulnerable to tyre 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 tyre problems during crash occurrence.

Thus, while tyre 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 tyres of the vehicle. This monitoring and maintenance can also provide safeguards against the emergence of tyre 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 tyre -related crashes.


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