III.  TIRE PRESSURE SURVEY AND TEST RESULTS

    Table of Contents

    In February 2001, the agency conducted a tire pressure study to determine the extent to which passenger vehicle operators are aware of the recommended air pressure for their tires, if they monitor air pressure, and to what extent the actual tire pressure differs from that recommended tire pressure by the vehicle manufacturer on the placard. The most useful information for this analysis is the snap shot in time that tells us where the actual tire pressure of the fleet is in comparison to the vehicle manufacturer’s recommended tire pressure. Although this was not a nationally representative survey, it is being treated as such in this analysis.

    The field data collection was conducted through the infrastructure of 24 locations of the National Automotive Sampling System Crashworthiness Data System (NASS CDS). Data were collected on 11,530 vehicles that were inspected at a sample of 336 gas stations. There were 6,442 passenger cars, 1,874 sport utility vehicles (SUVs), 1,376 vans, and 1,838 light conventional trucks. Data can be separated by passenger cars with P-metric tires; trucks, SUVs and vans with P-metric tires; and trucks, SUVs, and vans with either LT-type or high flotation tires. For this analysis we only compare the passenger car tire pressures and the light truck tire pressures, without separating the light trucks by type of tire. Complete data were collected on 5,967 passenger cars and 3,950 light trucks for a total of 9,917 vehicles. [10]

    The average placard pressure for passenger cars was about 30 psi, while the average placard pressure for light trucks was about 35 psi, although the light trucks have a much wider range of manufacturer recommended placard pressure. Because of the wide range of placard pressure for light trucks, it was determined that it would be best to propose a percentage reduction from the placard than a straight psi reduction.

    The issue addressed is how often drivers would get a warning from a low tire pressure monitoring system. Table III-1 shows how often a driver would be warned anytime one or more tires fell 25% below the placard recommended pressure. An estimated 26 percent of passenger cars and 29 percent of light trucks (an average of 27.5 percent of the passenger car and light truck drivers) would get a warning at 25% below the placard recommended pressure.

    Table III-2 shows the distribution of tire pressure when at least one tire is 25 percent or more below placard in terms of whether one, two, three, or all four tires were at least 25 percent below placard.

    At the time the survey was done, there were 207 million vehicles on the road. An estimated 57 million vehicles, have at least one tire 25 percent or more below placard at any time.


Table III-1
Percent of Vehicles That Would Get a Warning
  Passenger Cars Light Trucks
25% or more Below Placard 26% 29%

Table III-2
Distribution of the Number of Tires on Vehicles
That Have One or More Tires that is
25% or more Below Placard
Number of Tires
25% or more
Below Placard
Passenger Cars Percent Light Trucks Percent
1 880 55.9% 542 47.2%
2 399 25.3 313 27.3
3 139 8.8 145 12.6
4 157 10.0 148 12.9
Total 1,575 100% 1,148 100%

    TPMS Test Results

    The agency tested six direct measurement systems (Systems E through J in Table III-3) to determine both the level at which they provided driver information and the accuracy of the systems. The warning level thresholds were determined by dynamic testing at GVWR at 60 mph by slowly leaking out air out of one tire to a minimum of 14 psi. Some of the systems provide two levels of driver information, an advisory and a warning level. System F was a prototype with much lower thresholds for advisory and warning than the other systems. If System F is not considered, based on our testing, the typical advisory level is given at 20 percent under placard pressure, however the warning level averaged 36 percent below the placard. The static accuracy tests showed that those systems that displayed tire pressure readings were accurate to within 1 to 2 psi.


Table III-3
Direct measurement systems
Driver information provided at (%) below placard for one tire
System E F G H I J
Advisory N.A. -42% N.A. -20% N.A. -19%
Warning -20% -68% -33% -53% -35% -41%

    The agency tested four indirect measurement systems (Systems A to D) to determine when they provided driver information. The warning thresholds were determined by slowly leaking out air out of one tire to a minimum of 14 psi, while driving at 60 mph under a lightly loaded vehicle weight condition (LLVW) and at gross vehicle weight rating (GVWR). Table III-4 provides these results. The agency believes that the difference in the warning levels between the front and rear axle are due to variability in the system. The indirect systems could not detect when air was leaked out of different combinations of two tires and all four tires.


Table III-4
Indirect measurement systems
Driver warning provided at (%) below placard for one tire
Load Axle System A System B System C System D Ave. of 3
LLVW Front -21% No Warning -40% -28% -30%
LLVW Rear -16% No Warning -37% -38% -30%
GVWR Front -16% No Warning -18% -31% -24%
GVWR Rear -9% No Warning -20% N/a -14%

    Vehicle Stopping Distance Tests

    One of the potential safety benefits the agency is examining is the impact of low tire pressure on vehicle stopping distance. In the PRIA, we present two sets of data from different sources – Goodyear Tire and Rubber Company and NHTSA’s Vehicle Research and Test Center (VRTC). In a comment to the docket, Goodyear presented the results of additional testing. The information provided by these sources did not lead to the same conclusions.

    Table III-5 shows data provided by Goodyear on an ABS-equipped vehicle. These wet stopping distance data indicate:

    1. Stopping distance generally increases with lower tire pressure. The only exception was on concrete at 25 mph.

    2. With fairly deep water on the road, (0.050 inches is equivalent to 1 inch of rain in an hour) lowering inflation to 17 psi and increasing speed to 45 mph increases the potential for hydroplaning and much longer stopping distances.

    3. Except for 25 mph on macadam, the difference between 25 and 29 psi is relatively small.

    Goodyear provided test data to the agency on Mu values to calculate dry stopping distances. This information is used in the benefits chapter later in this assessment.


Table III-5
Braking Distance (in feet) provided by Goodyear
Wet Stopping Distance (0.050" water depth)
Surface Speed 17 psi 25 psi 29 psi 35 psi
Macadam 25 mph 32.4 30.8 29 27.4
Macadam 45 mph 107.6 101 100.8 98.6
Concrete 25 mph 47.4 48.2 48.2 48
Concrete 45 mph 182.6 167.2 167.4 163.6

    Table III-6 shows test data from NHTSA - VRTC on stopping distance. Tests were performed using a MY 2000 Grand Prix with ABS. Shown is the average stopping distance based on five tests per psi level. The concrete can be described as a fairly rough surface that has not been worn down like a typical road. The asphalt was built to Ohio highway specifications, but again has not been worn down by traffic, so it is like a new asphalt road. A wet road consists of wetting down the surface by making two passes with a water truck; thus it has a much lower water depth than was used in the Goodyear tests.


Table III-6
Braking Distance (in feet) from NHTSA testing
Stopping Distance from 60 mph
Surface 15 psi 20 psi 25 psi 30 psi 35 psi
Wet Concrete 148.8 147.5 145.9 144.3 146.5
Dry Concrete 142.0 143.0 140.5 140.4 139.8
Wet Asphalt 158.5 158.6 162.6 161.2 158.0
Dry Asphalt 144.0 143.9 146.5 148.2 144.0

    These stopping distances indicate:

    1. There is generally an increase in stopping distance as tire inflation decreases from the 30 psi placard on this vehicle on both wet and dry concrete.

    2. On wet and dry asphalt, the opposite generally occurs, stopping distance decreases as tire inflation decreases from the 30 psi placard.

    3. There is very little difference between the wet and dry stopping distance on the concrete pad (about 4 feet at 30 psi), indicating the water depth was not enough to make a noticeable difference on the rough concrete pad. There is a larger difference between the wet and dry stopping distance on the asphalt pad (13 feet at 30 psi).

    4. No hydroplaning occurred in the NHTSA tests, even though they were conducted at higher speed (60 mph vs. 45 mph in the Goodyear tests) and at lower tire pressure (15 psi vs. 17 psi in the Goodyear tests). Again, this suggests that the water depth in the VRTC tests was not nearly as deep as in the Goodyear testing.

    In general, these data suggest that the road surface and depth of water on the road have a large influence over stopping distance. Given a specific road condition, one can compare the difference in stopping distance when the tire inflation level is varied. The Goodyear test results imply that tire inflation can have a significant impact on stopping distance, while the NHTSA testing implies these impacts would be minor or nonexistent on dry surfaces and wet surfaces with very little water depth.

    In a comment to the docket (8572-160) Goodyear presented an extensive series of test data. These tests included two vehicles having tires with full tread depth and half tread depth on vehicles with ABS and on tires with full tread depth without ABS and on a dry, 0.02 inch wet and 0.05 inch wet macadam surface at three different psi levels. The full tread depth on the Integrity tire used on the Dodge Caravan was 10/32 inch and the half tread depth was 5/32 inch. The full tread depth on the Wrangler tire used on the Ford Ranger was 13/32 inch and the half tread depth was 6.5/32 inches. The stopping distance in feet is the average of six stops for most of the scenarios. The stopping distance was collected from 45 mph to 5 mph. Goodyear found that collecting the data at 5 mph reduced the variability in the results as compared to a full stop to 0 mph. Tables III-7 (a), (b), and (c) summarize these results.


Table III-7 (a)
Goodyear data – Second Test Series
Dry Macadam Surface
(Stopping Distance in Feet)
2001 Dodge Grand Caravan Sport 20 psi 28 psi 35 psi
Full Depth Tread with ABS 75.5 76.2 75.8
½ Depth Tread with ABS 69.9 68.1 66.3
       
Full Depth Tread without ABS 98.3 95.9 91.6
       
1997 Ford Ranger      
Full Depth Tread with ABS 80.8 78.2 77.6
½ Depth Tread with ABS 79.0 74.8 71.4
       
Full Depth Tread without ABS 97.8 96.5 94.1

Table III-7 (b)
Goodyear data – Second Test Series
0.02 Inch Wet Macadam Surface
(Stopping Distance in Feet)
2001 Dodge Grand Caravan Sport 20 psi 28 psi 35 psi
Full Depth Tread with ABS 79.8 78.5 77.1
½ Depth Tread with ABS 84.7 73.7 81.4
       
Full Depth Tread without ABS 111.1 110.2 108.6
       
1997 Ford Ranger      
Full Depth Tread with ABS 83.8 81.5 79.8
½ Depth Tread with ABS 91.5 89.4 84.6
       
Full Depth Tread without ABS 131.9 126.0 118.4

Table III-7 (c)
Goodyear data – Second Test Series
0.05 Inch Wet Macadam Surface
(Stopping Distance in Feet)
2001 Dodge Grand Caravan Sport 20 psi 28 psi 35 psi
Full Depth Tread with ABS 80.0 81.1 82.7
½ Depth Tread with ABS 103.7 99.7 92.2
       
Full Depth Tread without ABS 118.0 112.2 111.7
       
1997 Ford Ranger      
Full Depth Tread with ABS 89.7 86.0 81.5
½ Depth Tread with ABS 125.7 118.5 104.5
       
Full Depth Tread without ABS 142.9 134.8 125.7

    These data indicate that stopping distance is longer with lower psi for every case except for two cases with the full depth tread with ABS on the Dodge Caravan. Full depth tread tires had shorter stopping distance than ½ depth tread tires on wet surfaces, but not dry surfaces, and vehicles with ABS had shorter stopping distances than those vehicles without ABS.

    The value of Mu is dependent on surface material (concrete, asphalt, etc.), surface condition (wet vs. dry), inflation pressure, and initial velocity. The following tables presents coefficient of friction data provided by The Goodyear Tire and Rubber Company in response to the earlier NPRM, NHTSA developed a model that predicts Mu based on initial velocity and inflation pressure. Separate models were developed for Mu at both peak (the maximum level of Mu achieved while the tire still rotates under braking conditions) and slide (the level of Mu achieved when tires cease to rotate while braking (i.e., skid)). These models are used in the benefits section when estimating stopping distance.


GOODYEAR COEFFICIENT OF FRICTION DATA – M
Macadam Surface
1215/70R15 Integrity – 1080 lbs. Load
0.020" Wet 0.050" Wet DRY
  20 mph 20 mph 20 mph    
  35 psi 28 psi 20 psi 35 psi 28 psi 20 psi 35 psi 28 psi 20 psi
Peak 0.864 0.846 0.818 0.830 0.795 0.796 0.980 0.992 0.966
Slide 0.566 0.546 0.528 0.553 0.512 0.497 0.716 0.671 0.648
 
  0.020" Wet 0.050" Wet DRY  
  40 mph 40 mph 40 mph  
  35 psi 28 psi 20 psi 35 psi 28 psi 20 psi 35 psi 28 psi 20 psi
Peak 0.827 0.808 0.786 0.740 0.687 0.690 0.940 0.926 0.921
Slide 0.474 0.454 0.448 0.444 0.416 0.397 0.696 0.696 0.682
 
  0.020" Wet 0.050" Wet DRY    
  60 mph 60 mph 60 mph  
  35 psi 28 psi 20 psi 35 psi 28 psi 20 psi 35 psi 28 psi 20 psi
Peak 0.832 0.831 0.802 0.564 0.484 0.488 0.930 0.910 0.923
Slide 0.368 0.373 0.348 0.280 0.220 0.148 0.730 0.737 0.766

NHTSA - TIRE COEFFICIENT OF FRICTION DATA - m
Macadam Surface
P235/75R15 Wrangler RT/S  - 1490 lbs. Load
  0.020" Wet
20 mph
0.050" Wet
20 mph
DRY
20 mph
  35 psi 28 psi 20 psi 35 psi 28 psi 20 psi 35 psi 28 psi 20 psi
Peak 0.924 0.913 0.864 0.878 0.844 0.790 0.942 0.961 0.904
Slide 0.600 0.562 0.522 0.548 0.502 0.491 0.690 0.606 0.644
 
  0.020" Wet
40 mph
0.050" Wet
40 mph
DRY
40 mph
  35 psi 28 psi 20 psi 35 psi 28 psi 20 psi 35 psi 28 psi 20 psi
Peak 0.888 0.848 0.808 0.800 0.752 0.708 0.916 0.882 0.834
Slide 0.466 0.465 0.440 0.422 0.382 0.347 0.618 0.631 0.620
 
  0.020" Wet
60 mph
0.050" Wet
60 mph
DRY
60 mph
  35 psi 28 psi 20 psi 35 psi 28 psi 20 psi 35 psi 28 psi 20 psi
Peak 0.840 0.806 0.770 0.602 0.626 0.555 0.882 0.860 0.814
Slide 0.364 0.346 0.314 0.266 0.212 0.133 0.672 0.700 0.704


    IV. TARGET POPULATION

    Table of Contents

    Safety Problems Associated with Low Tire Pressure

    Under-inflation affects many different types of crashes. In commenting to the docket, the International Tire & Rubber Association (ITRA) (Docket No. 8572-123) stated that when developing ITRA training programs they look closely at tire performance and have the opportunity to analyze a significant number of tires that failed in service. ITRA has found that the single most common cause of tire failure is under-inflation.

    The types of crashes that under-inflation influences are:

    1. skidding and/or a loss of control of the vehicle in a curve, like an off-ramp maneuver coming off of a highway at high speed, or simply taking a curve at high speed
    2. skidding and/or loss of control of the vehicle in a lane change maneuver,
    3. hydroplaning on a wet surface, which can affect both stopping distance and skidding and/or loss of  control.
    4. an increase in stopping distance,
    5. crashes caused by flat tires and blowouts
    6. overloading the vehicle

    We can identify target populations for skidding and loss of control crashes, flat tires and blowouts, and stopping distance (which involves any vehicle that brakes during a crash sequence). We cannot identify from our crash files, or other reports, the incidence of hydroplaning specifically (we do however identify wet surfaces and loss of control in our "skidding and loss of control" analysis of crashes), or the impacts of overloading a vehicle (this may be captured somewhat in tire blowouts).

    Skidding and loss of control

    The 1977 Indiana Tri-level study associated low tire pressure with loss of control, on both wet and dry pavements. That study did not identify low tire pressure as a "definite" (95 percent certain that the crash would not have occurred without this cause) cause of any crash, but did identify it as a "probable" cause (80 percent confidence level  - highly likely that the crash would not have occurred) of the crash in 1.4 percent of the 420 in-depth crash investigations. [11]

    "Probable cause" was broken up into two levels: a causal factor and a severity-increasing factor. A causal factor was defined as "had the factor not been present in the accident sequence, the accident would not have occurred." A severity-increasing factor was not sufficient to result in the occurrence of the accident, but resulted in an increase in speed of the initial impact. Under-inflated tires were a causal factor in 1.2 percent of the probable causes and a severity-increasing factor in 0.2 percent of the probable causes.

    Note that more than one "probable cause" could be assigned to a crash. In fact, there were a total of 138.8 percent causes listed as probable cause (92.4 percent human factors, 33.8 percent environmental factors, and 12.6 percent vehicular factors). Thus, under-inflation’s part of the total is 1.0 percent (1.4/138.8). If we focus on just the probable cause cases, under-inflation represents 0.86 percent of crashes (1.2/1.4*1.0).

    There are several important factors to know about the Indiana Tri-Level study and their implications for this analysis. This information was verified with the authors of the study and NHTSA contract technical managers on the study.

    1)      None of the cases in which under-inflation was cited as a probable cause dealt with stopping distance. They were all cases of loss of control in a curve or in a crash avoidance maneuver.

    2)      High speed was not a factor in these cases. In order to be considered for an under-inflation case, the vehicle had to be going within a reasonable speed to make the turn, for example.

    3)      In order for under-inflation to be cited in the study, there had to be a significant amount of under-inflation, 10 to 15 psi low or more compared to placard levels. Thus, the estimates would apply to all three Compliance Options fairly equally.

    4)      There were particular vehicles that were known to lose traction when their tires were under-inflated in particular patterns, sometimes the rear tires, or sometimes a disparity in inflation. The authors particularly noted the Chevrolet Corvair and the early-60’s Volkswagen Beetle. Problem vehicles like this were not a big part of the sample but raised the rate somewhat and do not appear to be a problem today. We assume this factor could reduce the probable cause estimate by 10 percent to 0.77 (0.86*.9).

    At the time of the study, radial tires were on 12% of passenger vehicles, and now they are on more than 90% of passenger vehicles, including all tires on new automobiles. The question is whether the 1977 results are applicable in today’s tire environment. The agency at this time is unable to quantify how the cornering force capability of different tire constructions (bias ply, bias belted and radial) at different tire inflation pressures affects the frequency of loss of control crashes. Radial tires provide better tread contact with the pavement since their sidewalls are more flexible in the lateral direction than bias ply tires. Accordingly, radial tires can generate about twice the lateral force as bias ply tires. However, drivers get feedback from their tires and drive vehicles with different types of tires in different ways around corners. Bias and bias belted tires provide more feedback to the driver by feel and noise that the vehicle might not negotiate a curve, and the driver can sometimes slow down and correct the situation before going off the road. While radial tires generate more lateral forces, they do not provide progressive feedback to the driver and tend to lose traction without as much warning. In essence, drivers have learned how to go around entrance and exit ramps, and other curves, on highways at a higher rate of speed with radial tires. However, if the road is wet and their tire pressure is low, then they might have problems taking that curve at the same speed. Thus, we can’t determine how to correct the Indiana Tri-Level study to account for the difference in types of tires. It may well be, and for this assessment we assume, that the same percentage of under-inflation influenced crashes occur with radial tires as with bias and bias-belted tires.

    To get an estimate of the target population of the low tire pressure cases in which skidding and loss of control could be a factor, we took data from "Traffic Safety Facts, 1999" which shows there were about 47,848 passenger vehicles (passenger cars and light trucks) involved in fatal crashes, about 3.6 million passenger vehicles involved in injury crashes and about 6.9 million passenger vehicles involved in property damage only crashes. These crashes resulted in 32,061 passenger vehicle occupants being killed and almost 3 million passenger vehicle occupants being injured.

    Taking 0.77 percent of these cases, loss of control and skidding due to low tire pressure would account for an estimated 247 occupants killed, 23,100 occupants injured, and 53,130 property damage only crashes.

    As a second check on these estimates, the 1999 NASS-GES was examined to identify particular crash scenarios in which loss of control occurred. The following scenarios that could be identified were examined totaling over 413,000 vehicles (3.9 percent of the vehicles in all crashes). Certainly there are other scenarios that couldn’t be identified, but this check was made to assure ourselves that 0.77 percent was not impossibly high, which it did.

    Negotiating a curve: Where the vehicle left the roadway, left the travel lane, lost control or skidded (213,759 vehicles)

    Changing lanes where the vehicle left the roadway, lost control or skidded (4,890 vehicles), and Raining/wet road cases where the vehicle lost control and skidded (194,709 vehicles).

    Flat tires and blowouts

    There is no direct evidence in NHTSA’s current crash files (FARS and NASS) that points to low tire pressure as the cause of a particular crash. This is because we have no measurements of tire pressure in our data bases (plans are underway to start collecting this data in 2002). The closest data element is "flat tire or blowout". Even in these cases, crash investigators cannot tell whether low tire pressure contributed to the tire failure. Tire failures, especially blowouts, are associated with rollover crashes. Low tire pressure can also lead to loss of control or a skid initially. Skids can lead to tripping and then to a rollover.

    The agency examined its crash files to gather whatever information is available on tire-related problems causing crashes. The National Automotive Sampling System - Crashworthiness Data System (NASS-CDS) has trained investigators who collect data on a sample of tow-away crashes around the country. These data can be weighted up to national estimates. The NASS-CDS contains on its General Vehicle Form a space for the following information (where applicable): 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. It does not include cases where there was improper tire pressure in one or more tires that did not allow the vehicle to handle as well as it should have in an emergency situation. This coding would only be used when the tire went flat or there was a blowout and caused a loss of control of the vehicle, resulting in a crash. However, as stated above, low tire pressure may contribute directly to the crashes discussed in the paragraphs below. In addition, there may be other crashes, not included in the paragraphs below, where low tire pressure played a part.

    NASS-CDS data for 1995 through 1998 were examined and average annual estimates are provided below in Table IV-1. Table IV-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 a percent of all crashes are caused by these tire problems. When these cases are broken down by passenger car versus light truck, and compared to the total number of crashes for passenger cars and light trucks individually, it is found that blowouts cause more than three times the rate of crashes in light trucks (0.99 percent) than in passenger cars (0.31 percent). When the data are further divided into rollover versus non-rollover, blowouts cause a much higher proportion of rollover crashes (4.81) than non-rollover crashes (0.28); and again more than three times the rate in light trucks (6.88 percent) than in passenger cars (1.87 percent).


Table IV-1
Estimated Annual Average Number and Rates of
Blowouts or Flat Tires Causing Tow-away Crashes
  Tire Related Cases Percent Tire Related
Passenger Cars Total 10,170 0.31%
Rollover 1,837 (18%) 1.87%
Non-rollover 8,332 (82%) 0.26%
     
Light Trucks Total 13,294 0.99%
Rollover 9,577 (72%) 6.88%
Non-rollover 3,717 (28%) 0.31%
     
Light Vehicles Total 23,464 0.51%
Rollover 11,414 (49%) 4.81%
Non-rollover 12,049 (51%) 0.28%

    Table IV-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 [12]. There are an estimated 414 fatalities and 10,275 non-fatal injuries in these crashes.


Table IV-2
Injuries/Fatalities in Crashes Caused by
Flat Tire/Blowout
  Non-fatal
AIS 1
Non-fatal
AIS 2
Non-fatal
AIS 3
Non-fatal
AIS 4
Non-fatal
AIS 5
Fatalities
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 IV-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 time, followed by light trucks in northern states in the summer time, and by passenger cars in southern states in the summertime. It thus appears that tire problems are heat related.


    Table IV-3
    Geographic and Seasonal Analysis of Tire Problems
    (Percent of Vehicles in) FARS with Tire Problems

      Passenger
    Cars
    Light
    Trucks
    All Light
    Vehicles
    Northern States      
    Winter 1.01% 0.80% 0.94%
    Spring 1.12% 1.01% 1.08%
    Summer 0.98% 1.46% 1.15%
    Fall 1.04% 0.93% 1.00%
           
    Southern States      
    Winter 0.87% 0.99% 0.92%
    Spring 1.09% 1.27% 1.16%
    Summer 1.31% 1.99% 1.59%
    Fall 0.89% 1.07% 1.00%

     

     

    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.

     

     

    There are also crashes indirectly caused or indirectly involved with 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 see what is going on. This can create congestion, potentially resulting in a rear-end impact later in the line of vehicles when some driver isn’t paying enough attention to the traffic in front of them. The agency has not attempted to estimate how often a TPMS would give the driver enough warning of an impending flat tire that they could have the tire repaired before they get stuck having to repair a flat tire in traffic. However, it should be a very large number.

    An indirectly involved crash relating to tire repairs on the road can occur when someone is in the act of changing a tire on the shoulder of the road. Sometimes drivers repairing 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 IV-4.


Table IV-4
State data on tire problems and pedestrians
  Ohio Washington Pennsylvania
Pedestrians Injured 3,685 2,068 5,226
Pedestrians Injured While Playing or Working on Vehicle 50
(1.4%)
27
(1.3%)
56
(1.1%)
Pedestrians Injured While Working on Vehicle with Tire Problem 0 2 0
       
Total Crashes 385,704 140,215 144,169
Crashes with Tire Problems 862
(0.22%)
1,444
(1.03%)
794
(0.55%)

    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 IV-1 of 0.51 percent. The number 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. It is possible that these numbers could be much higher, if they were coded correctly. The agency is not going to estimate how many of the pedestrian injuries could be reduced with a TPMS.



    [10] The Rubbers Manufacturers Association (Docket 8572-116) argued the tire pressure survey measured tires when they were hot.  Thus, NHTSA’s under-inflation estimates are conservative.  The agency considered this point, but also notes that the survey was done in February when tires lose more pressure because of the ambient temperature and considered these to be unquantifiable offsetting conditions. 

    [11] Tri-level Study of the Causes of Traffic Accidents:  Executive Summary,  Treat, J.R., Tumbas, N.S., McDonald, S.T., Shinar, D., Hume, R.D., Mayer, R.E., Stansifer, R.L., & Castellan, N.J. (1979).  (Contract No. DOT HS 034-3-535). DOT HS 805 099. Washington, DC: U.S. Department of Transportation, NHTSA.  See pages A-51 and D-23 to D-30. 

    [12] Since CDS typically underestimates the number of fatalities, a factor was developed based on the number of occupant fatalities in FARS divided by the number of occupant fatalities in CDS for those years of 1.163, which was multiplied by the actual estimate of flat tire/blowout fatalities.


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