III. TARGET POPULATION
Safety Problems Associated With Tires
There is no direct evidence in NHTSA's crash data files that points to defective or sub-standard tires as the cause of a particular crash. The closest data element is "flat tire or blowout". Even in these cases, crash investigators do not record what caused the tire failure. Tire failures, especially blowouts, are typically associated with rollover crashes.
It is possible that a combination of lesser quality tires (lesser quality being defined here as designs that do not adequately dissipate heat, which causes the tire to rapidly build-up heat which ultimately causes the tire failure) being operated in an under-inflated state and/or an overloaded state could account for many of the tire failures, since both under-inflation and overloading increase heat build-up in the tire. Severe under-inflation coupled with an emergency steering maneuver could cause the tire to "de-bead," i.e., separate from the rim, which could "trip" the vehicle and cause it to roll over.
The Target Population for General Tire-Related Crashes
The agency examined its crash files to gather available information on tire-related problems causing crashes. The 1977 Indiana Tri-level study investigated 2,258 crashes on-site and 420 crashes in-depth and found 3 cases (0.1 percent) where tire blowout was a certain or probably cause of the crash. However, there is no information as to what caused the blowout in the crash investigations. (1) At the time of the study, radial tires represented only 12% of the tire population and now they are more than 90%, including all tires on new light vehicles. Therefore, the 1977 results may not be applicable in today's tire environment.
The National Automotive Sampling System - Crashworthiness Data System (NASS-CDS) has trained investigators that collect data on a sample of tow-away crashes around the country. These data can be extrapolated to national estimates. The NASS-CDS contains on its General Vehicle Form the following information: a critical pre-crash event, vehicle loss of control due to a blowout or flat tire. This category only includes part of the tire-related problems causing crashes. This coding would only be used when the tire went flat rapidly or there was a blowout which caused a loss of control of the vehicle, resulting in a crash.
NASS-CDS data for 1995 through 1998 (with predominately radial tires) were examined and average annual estimates are provided below in Table III-1. Table III-1 shows that there are an estimated 23,464 tow-away crashes caused per year by blowouts or flat tires. Thus, about one half of one percent of all crashes are caused by these tire problems. The denominator for the right hand column of Table III-1 is all crashes by the vehicle type in the row. When these cases are broken down by passenger cars versus light trucks, blowouts cause more than three times the number of crashes in light trucks (0.99 percent) than in passenger cars (0.31 percent). Blowouts cause a much higher proportion of rollover crashes (4.81 percent) than non-rollover crashes (0.28 percent); and the rate in light trucks (6.88 percent) is more than three times the rate in passenger cars (1.87 percent).
Estimated Annual Average Number (1995-98 NASS) and Rates of
Blowouts or Flat Tires Causing Tow-away Crashes
|Tire Related Cases||Percent Tire Related|
|Passenger Cars Total||10,170||0.31%|
|Light Trucks Total||13,294||0.99%|
|Light Vehicles Total||23,464||0.51%|
Table III-2 shows the estimated number of fatalities and injuries in those cases in which a flat tire/blowout was considered the cause of the crash (2). There are an estimated 414 fatalities and 10,275 non-fatal injuries in these crashes.
We examined these crashes by speed limit of the highway, knowing that the heat build-up is related to speed. Of the 414 fatalities, 306 (74 percent) occurred on highways with posted speed limits of 55 mph or higher. Of the 10,275 injuries, 6,590 (64 percent) occurred on highways with posted speed limits of 55 mph or higher.
Injuries/Fatalities in Crashes Caused by
|Number of Injuries||8,231||1,476||362||155||51||414|
The Fatality Analysis Reporting System (FARS) was also examined for evidence of tire problems involved in fatal crashes. In the FARS system, tire problems are noted after the crash, if they are noted at all, and are only considered as far as the existence of a condition. In other words, in the FARS file, we don't know whether the tire problem caused the crash, influenced the severity of the crash, or just occurred during the crash. For example, (1) some crashes may be caused by a tire blowout, (2) in another crash, the vehicle might have slid sideways and struck a curb, causing a flat tire which may or may not have influenced whether the vehicle rolled over. Thus, while an indication of a tire problem in the FARS file gives some clue as to the potential magnitude of the tire problem in fatal crashes, it can neither be considered the lowest possible number of cases nor the highest possible number of cases. In 1995 to 1998 FARS, 1.10 percent of all light vehicles were coded with tire problems. Light trucks had slightly higher rates of tire problems (1.20 percent) than passenger cars (1.04 percent). The annual average number of vehicles with tire problems in FARS was 535 (313 in passenger cars and 222 in light trucks). On average, annually there were 647 fatalities in these crashes (369 in passenger cars and 278 in light trucks). Thus, these two sets of estimates seem reasonably consistent: 647 fatalities in FARS in crashes in which there was a tire problem and 414 fatalities from CDS, in which the flat tire/blowout was the cause of the crash.
Geographic and Seasonal Effects
The FARS data were further examined to determine whether heat is a factor in tire problems (see Table III-3). Two surrogates for heat were examined: (1) in what part of the country the crash occurred, and (2) in what season the crash occurred. The highest rates occurred in light trucks in southern states in the summer, followed by light trucks in northern states in the summer, and by passenger cars in southern states in the summer. The lowest rates occurred in winter and fall. The denominator is all passenger cars or light trucks in fatal crashes by season. It thus appears that tire problems are heat related.
Geographic and Seasonal Analysis of Tire Problems
(Percent of Vehicles) in FARS with Tire Problems
|Passenger Cars||Light Trucks||All Light Vehicles|
Winter = December, January, February.
Spring = March, April, May
Summer = June, July, August
Fall = September, October, November.
Southern States = AZ, NM, OK, TX, AR, LA, KY, TN, NC, SC, GA, AL, MS, and FL.
Northern States = all others.
Tire Problems by Tire Type and Light Truck Type
The agency also examined tire problems in the NASS-CDS database from 1992 to 1999 by types of light trucks and vehicle size to determine whether LT tires used on light trucks had more tire problems than P-metric tires. Table III-4 provides the results of this analysis, showing the unweighted number of cases. The unweighted numbers are used since in this case, as sometimes happens when NASS data are broken up into a small number of cells, the results obtained using weighted numbers do not appear to be logical.
LT tires are used on the vehicle classes we have identified for this analysis as Van Large B and Pickup Large B groups of vehicles. These groups of vehicles typically represent the ¾-ton and 1-ton vans and pickups. P-metric tires are used on most other light trucks. The data indicate that the average percent of the light trucks in NASS-CDS that have an LT tire problem is 0.84 percent (10/1,186), while the average percent of the light trucks that have a P-metric tire problem is 0.47 percent (53/11,226). Of course larger pickups and vans are also the vehicles that carry the heavier loads and may be more likely to be overloaded than other light trucks. In addition, these heavier vehicles are often used at construction sites and may be more apt to pick up nails resulting in flat tires. Thus, there may well be driver behavior issues that drive the percentage of tire problems up for these larger trucks, rather than any qualitative difference between P-metric and LT tires.
Tire Problems by Light Truck Vehicle Type
1992 to 1999 NASS-CDS Data
|Light Truck Type||Number of Cases
with a Tire
Cases with a
|Van - Compact||11||2,125||0.52|
|Van - Large A||3||431||0.70|
|Van - Large B||4||501||0.80|
|Pickup - Compact||13||3,155||0.41|
|Pickup - Large A||7||1,849||0.38|
|Pickup - Large B||6||685||0.88|
|SUV - Compact||16||3,147||0.51|
|SUV - Large||3||519||0.58|
The Van - Large A group includes vehicles like the Ford Econoline - 150
The Van - Large B group includes vehicles like the Ford Econoline - 250/350
The Pickup - Large A group includes vehicles like the Ford F-150
The Pickup - Large B group includes vehicles like the Ford F-250/350
Crashes Indirectly Caused by Tire Problems
There are also crashes indirectly caused by tire related problems. If a vehicle stops on the side of the road due to a flat tire, there is the potential for curious drivers to slow down to determine the reason for the stopped vehicle. This can create congestion, potentially resulting in a rear end impact further back in the line of vehicles when some driver isn't paying enough attention to the traffic in front of him/her
Another crash type indirectly caused by tire problems involve crashes relating to incidents on the road when a driver is in the act of changing a tire on the shoulder of the road. Sometimes drivers changing tires are struck (as pedestrians) by other vehicles. This phenomena is not captured in NHTSA's data files, but there are three states (Pennsylvania, Washington, and Ohio) which have variables in their state files which allow you to search for and combine codes such as "Flat tire or blowout" with "Playing or working on a vehicle" with "Pedestrians". An examination of these files for calendar year 1999 for Ohio and Pennsylvania and for 1996 for Washington found the following information shown in Table III-5.
State data on tire problems and pedestrians
|Pedestrians Injured While Playing or Working on Vehicle||50
|Pedestrians Injured While Working on Vehicle with Tire Problem||0||2||0|
|Crashes with Tire Problems Not Coded in GES||862
The combined percent of total crashes with tire problems of these three states (3,100/670,088 =
0.46 percent) compares very favorably with the NASS-CDS data presented in Table III-1 of 0.51
percent. The portion of pedestrians coded as being injured while working on a vehicle with tire
problems is 2/10,979 = 0.018 percent. Applying this to the estimated number of pedestrians
injured annually across the U.S. (85,000 from NASS-GES), results in an estimated 15
pedestrians injured per year due to tire problems. The agency does not have data to estimate how
many of the pedestrian injuries could be reduced by having better tires.
2 Since CDS typically underestimates the number of fatalities, a factor of 1.163 was developed based on the number of occupant fatalities in FARS divided by the number of occupant fatalities in CDS for those years. The actual estimate of flat tire/blowout fatalities were multiplied by the1.163 factor.