III. Safety Problem
Many vehicles have significantly under-inflated tires, primarily because drivers infrequently check their vehicles' tire pressure. Other contributing factors are the difficulty of visually detecting when a tire is significantly under-inflated and the loss of tire pressure due to natural leakage and seasonal climatic changes.
A. Infrequent Driver Monitoring of Tire Pressure
Surveys have shown that most drivers check the inflation pressure in their vehicles' tires infrequently. For example, in September 2000, the Bureau of Transportation Statistics (BTS) conducted an omnibus survey for NHTSA. One of the questions posed was: "How often do you, or the person who checks your tires, check the air pressure in your tires?" The answers indicated that 29 percent of the respondents stated that they check the air pressure in their tires monthly; another 29 percent stated that they check the air pressure only when one or more of their vehicle's tires appears under-inflated; 19 percent stated that they only have the air pressure checked when the vehicle is serviced; 5 percent stated that they only check the air pressure before taking their vehicle on a long trip; and 17 percent stated that they check the air pressure on some other occasion. Thus, 71 percent of the respondents stated that they check the air pressure in the vehicles' tires less than once a month. (17)
In addition, NHTSA's National Center for Statistics and Analysis (NCSA) conducted a survey in February 2001. The survey was designed to assess the extent to which passenger vehicle drivers are aware of the recommended air pressure for their vehicles' tires, if drivers monitor air pressure, and to what extent actual tire pressure differs from placard pressure.
Data was collected through the infrastructure of the National Accident Sampling System - Crashworthiness Data System (NASS-CDS). The NASS-CDS consists of 24 Primary Sampling Units (PSUs) located across the country. Within each PSU, a random selection of zip codes was obtained from a list of eligible zip codes. Within each zip code, a random selection of two gas stations was obtained.
A total of 11,530 vehicles were inspected at these gas stations. This total comprised 6,442 passenger cars, 1,874 sports utility vehicles (SUVs), 1,376 vans, and 1,838 pick-up trucks. For analytical purposes, the data were divided into three categories: (1) passenger cars; (2) pick-up trucks, SUVs, and vans with P-metric tires; and (3) pick-up trucks, SUVs, and vans with either light truck (LT) or flotation tires.
Drivers were asked how often they normally check their tires to determine if they are properly inflated. Their answers are in the following table:
|How often is tire
passenger cars (%)
|Drivers of pick-up trucks, SUVs,|
and vans (%)
|P-metric tires||LT or flotation tires|
|When they seem low||25.63||23.58||15.59|
|For long trip||0.99||2.39||2.17|
|Do not check||6.56||4.16||1.69|
These data indicate that only about 30 percent of drivers of passenger cars, 34 percent of drivers of pick-up trucks, SUVs, and vans with P-metric tires, and 48 percent of drivers of pick-up trucks, SUVs, and vans with either LT or flotation tires claim that they check the air pressure in their vehicles' tires at least once a month.
B. Loss of Tire Pressure Due to Natural and Other Causes
According to data from the tire industry, 85 percent of all tire air pressure losses are the result of slow leaks that occur over a period of hours, days, or months. Only 15 percent are rapid air losses caused by contact with a road hazard, e.g., when a large nail that does not end up stuck in the tire punctures a tire.
Slow leaks may be caused by many factors. Tire manufacturers commented that tires typically lose air pressure through natural leakage and permeation at a rate of about 1 psi per month. Testing by CU supports those comments. In addition, tire manufacturers said that seasonal climatic changes result in air pressure losses on the order of 1 psi for every 10 degree F decrease in the ambient temperature. Slow leaks also may be caused by slight damage to a tire, such as a road hazard that punctures a small hole in the tire or a nail that sticks in the tire. NHTSA has no data indicating how often any of these causes results in a slow leak.
C. Percentage of Motor Vehicles with Under-Inflated Tires
During the February 2001 survey, NASS-CDS crash investigators measured tire pressure on each vehicle coming into the gas station and compared the measured pressures to the vehicle's placard pressure. They found that about 36 percent of passenger cars and about 40 percent of light trucks had at least one tire that was at least 20 percent below the placard pressure. (18) About 26 percent of passenger cars and 29 percent of light trucks had at least one tire that was at least 25 percent below the placard pressure. The agency notes those levels of under-inflation because they are the threshold levels for the low-tire pressure warning telltale illumination under the two alternatives the agency proposed in the NPRM for TPMSs. (66 FR 38982, July 26, 2001).
D. Consequences of Under-Inflation of Tires
1. Reduced Vehicle Safety -- Tire Failures and Increases in Stopping Distance
When a tire is used while significantly under-inflated, its sidewalls flex more and the air temperature inside the tire increases, increasing stress and the risk of failure. In addition, a significantly under-inflated tire loses lateral traction, making handling more difficult. Under-inflation also plays a role in crashes due to flat tires and blowouts. Finally, significantly under-inflated tires can increase a vehicle's stopping distance.
NHTSA's current crash files do not contain any direct evidence that points to low tire pressure as the cause of any particular crash. (19) However, this lack of data does not imply that low tire pressure does not cause or contribute to any crashes. The agency believes that it simply reflects the fact that measurements of tire pressure are not among the vehicle information included in the crash reports received by the agency and placed in its crash data bases. (20)
The only tire-related data element in the agency's crash databases is "flat tire or blowout." However, even in crashes for which a flat tire or blowout is reported, crash investigators cannot tell whether low tire pressure contributed to the tire failure.
The agency examined its crash files to gather information on tire-related problems that resulted in crashes. The NASS-CDS has trained investigators who collect data on a sample of tow-away crashes around the United States. These data can be weighted to generate national estimates.
The NASS-CDS General Vehicle Form contains a value indicating vehicle loss of control due to a blowout or flat tire. This value is used only when a vehicle's tire went flat, causing a loss of control of the vehicle and a crash. The value is not used for cases in which one or more of a vehicle's tires were under-inflated, preventing the vehicle from performing as well as it could have in an emergency situation.
NHTSA examined NASS-CDS data for 1995 through 1998 and estimated that 23,464 tow-away crashes, or 0.5 percent of all crashes, are caused by blowouts or flat tires each year. The agency placed the tow-away crashes from the NASS-CDS files into two categories: passenger car crashes and light truck crashes. Passenger cars were involved in 10,170 of the tow-away crashes caused by blowouts or flat tires, and light trucks were involved in the other 13,294.
NHTSA also examined data from the Fatality Analysis Reporting System (FARS) for evidence of tire problems in fatal crashes. In FARS, if tire problems are noted after the crash, the simple fact of their existence is all that is noted. No attempt is made to ascribe a role in the crash to those problems. Thus, the agency does not know whether the noted tire problem caused the crash, influenced the severity of the crash, or simply occurred during the crash. For example, a tire may have blown out and caused the crash, or it may have blown out during the crash when the vehicle struck some object, such as a curb.
Thus, while an indication of a tire problem in the FARS file gives some clue as to the potential magnitude of tire problems in fatal crashes, the FARS data cannot give a precise measure of the causal role played by those problems. The very existence of tire problems is sometimes difficult to detect and code accurately. Further, coding practices vary from State to State. Nevertheless, the agency notes that, from 1995 to 1998, 1.1 percent of all light vehicles involved in fatal crashes were coded as having tire problems. Over 535 fatal crashes involved vehicles coded with tire problems.
Under-inflated tires can contribute to types of crashes other than those resulting from blowouts or tire failure, including crashes which result from: skidding and/or a loss of control of the vehicle in a curve or in a lane change maneuver; an increase in a vehicle's stopping distance; or hydroplaning on a wet surface.
The 1977 Indiana Tri-level study associated low tire pressure with loss of control on both wet and dry pavements. The study never defined low tire pressure as a "definite" (i.e., 95 percent certainty that the crash would not have occurred absent this condition) cause of any crash, but did identify it as a "probable" (80 percent certainty that the crash would not have occurred absent this condition) cause of the crash in 1.4 percent of the 420 in-depth crash investigations.
The study divided "probable" cause into two levels: a "causal" factor and a "severity-increasing" factor. A "causal" factor was defined as a factor whose absence would have prevented the accident from occurring. A "severity-increasing" factor was defined as a factor whose presence was not sufficient, by itself, to result in the occurrence of the accident, but which resulted in an increase in speed of the initial impact. The study determined that under-inflated tires were a causal factor in 1.2 percent of the probable cause cases and a severity-increasing factor in 0.2 percent of the probable cause cases.
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 causes (92.4 percent human factors, 33.8 percent environmental factors, and 12.6 percent vehicle factors). Thus, tire under-inflation's part of the total is one percent (1.4/138.8). The agency focused solely on the probable cause cases, which represent 0.86 percent of crashes (1.2/1.4 * 1.0).
Tires are designed to maximize their performance capabilities at a specific inflation pressure. When a tire is under-inflated, the shape of its footprint and the pressure it exerts on the road surface are both altered, especially on wet surfaces. An under-inflated tire has a larger footprint than a properly inflated tire. Although the larger footprint results in an increase in rolling resistance on dry road surfaces due to increased friction between the tire and the road surface, it also reduces the tire load per unit area. On dry road surfaces, the countervailing effects of a larger footprint and reduced load per unit of area nearly offset each other, with the result that the vehicle's stopping distance performance is only mildly affected by under-inflation.
On wet surfaces, however, under-inflation typically increases stopping distance for several reasons. First, as noted above, the larger tire footprint provides less tire load per area than a smaller footprint. Second, since the limits of adhesion are lower and achieved earlier on a wet surface than on a dry surface, a tire with a larger footprint, given the same load, is likely to slide earlier than the same tire with a smaller footprint because of the lower load per footprint area. The rolling resistance of an under-inflated tire on a wet surface is greater than the rolling resistance of the same tire properly-inflated on the same wet surface. This is because the slightly larger tire footprint on the under-inflated tire results in more rubber on the road and hence more friction to overcome. However, the rolling resistance of an under-inflated tire on a wet surface is less than the rolling resistance of the same under-inflated tire on a dry surface because of the reduced friction caused by the thin film of water between the tire and the road surface. The less tire load per area and lower limits of adhesion of an under-inflated tire on a wet surface are enough to overcome the increased friction caused by the larger footprint of the under-inflated tire. Hence, under-inflated tires cause longer stopping distance on wet surfaces than properly-inflated tires.
The agency has received data from Goodyear indicating that significantly under-inflated tires increase a vehicle's stopping distance. (21) The effects of tire under-inflation on vehicle stopping distance are discussed in greater detail in the agency's Final Economic Analysis (FEA).
As explained in the FEA, the agency did not use the VRTC data or the Goodyear data that the agency used to estimate benefits in the NPRM because of concerns with the way in which the both tests were performed. (22) The agency believes that the more recent Goodyear test methodology adequately addressed these concerns. (23)
2. Reduced Tread Life
Unpublished data submitted to the agency by Goodyear indicate that when a tire is under-inflated, more pressure is placed on the shoulders of the tire, causing the tread to wear incorrectly. (24) The Goodyear data also indicate that the tread on an under-inflated tire wears more rapidly than it would if the tire were inflated to the proper pressure.
The Goodyear data indicate that the average tread life of a tire is 45,000 miles, and the average cost of a tire is $61 (in 2000 dollars). Goodyear also estimated that a tire's average tread life would drop to 68 percent of the expected tread life if tire pressure dropped from 35 psi to 17 psi and remained there. Goodyear assumed that this relationship was linear. Thus, for every 1-psi drop in tire pressure, tread life would decrease by 1.78 percent (32 percent / 18 psi). This loss of tread life would take place over the lifetime of the tire. Thus, according to Goodyear's data, if the tire remained under-inflated by 1 psi over its lifetime, its tread life would decrease by about 800 miles (1.78 percent of 45,000 miles).
As noted above, data from the NCSA tire pressure survey indicate that 26 percent of passenger cars had at least one tire that was under-inflated by at least 25 percent. The average level of under-inflation of the four tires on passenger cars with at least one tire under-inflated by at least 25 percent was 6.8 psi. Thus, on average, these passenger cars could lose about 5,440 miles (6.8 psi under-inflation x 800 miles) of tread life due to under-inflation, if their tires were under-inflated to that extent throughout the life of the tires.
Also as noted above, data from the NCSA tire pressure survey indicate that about 29 percent of light trucks had at least one tire that was under-inflated by at least 25 percent. The average level of under-inflation of the four tires on light trucks with at least one tire under-inflated by at least 25 percent was 8.7 psi. Thus, on average, these light trucks could lose about 6,960 miles (8.7 psi under-inflation x 800 miles) of tread life due to under-inflation, if their tires were under-inflated to that extent throughout the life of the tires.
3. Reduced Fuel Economy
Under-inflation increases the rolling resistance of a vehicle's tires and, correspondingly, decreases the vehicle's fuel economy. According to a 1978 report, fuel efficiency is reduced by one percent for every 3.3 psi of under-inflation. (25) More recent data provided by Goodyear indicate that fuel efficiency is reduced by one percent for every 2.96 psi of under-inflation. (26)
NHTSA notes that there is an apparent conflict between these data, which indicate that under-inflation increases rolling resistance and thus decreases fuel economy and the previously mentioned Goodyear data that indicates under-inflated tires increase a vehicle's stopping distance. While an under-inflated tire typically has a larger tread surface area (i.e., tire footprint) in contact with the road, which might be thought to improve its traction during braking, the larger tire footprint also reduces the tire load per unit area. The larger footprint does result in an increase in rolling resistance on dry road surfaces due to increased friction between the tire and the road surface. On dry road surfaces, though, the countervailing effects of a larger footprint and reduced load per unit of area nearly offset each other, with the result that the vehicle's stopping distance performance is only mildly affected by under-inflation on those surfaces. However, as explained above in section III.D.1., "Reduced Vehicle Safety -- Tire Failures and Increases in Stopping Distance," on wet surfaces other attributes of under-inflation lead to increased stopping distances.
18 For purposes of this discussion, the agency classified pick-up trucks, SUVs, and vans with either P-metric, LT, or flotation tires as light trucks.
19 In response to the TREAD Act, NHTSA has added new tire related variables and attributes, including tire make, model, recommended tire pressure, actual tire pressure, and tread depth to its crash databases. These new variables will provide more specific tire data for vehicles involved in crashes.
20 These crash databases are the NASS-CDS and the Fatality Analysis Reporting System (FARS).
21 Goodyear submitted these data to the docket in a letter dated September 14, 2001. See Docket No. NHTSA-2000-8572-160. OMB criticized NHTSA's application of these data to certain vehicle types in estimating safety benefits for this rulemaking. The agency responds to that criticism below in section VI.F., "Technical Foundation for NHTSA's Safety Benefit Analyses." The Alliance also questioned NHTSA's use of the Goodyear data. The agency explains its use of the Goodyear data below in footnotes 22 and 23, and in the agency's Final Economic Analysis (FEA).
22 For example, the VRTC only tested new tires, not worn tires that are more typical of the tires on most vehicles. In addition, the NHTSA track surface is considered to be aggressive in that it allows for maximum friction with tire surfaces. It is more representative of a new road surface than the worn surfaces experienced by the vast majority of road traffic. The previous Goodyear tests on wet surfaces were conducted on surfaces with .05 inch of standing water. This is more than would typically be encountered under normal wet road driving conditions. The agency expressed concerns with the adequacy of both sets of test data in a memo to the docket. (Docket No. NHTSA-2000-8572-81.)
23 For example, in its more recent tests Goodyear tested tires with two tread depths: full tread, which is representative of new tires, and half tread, which is representative of worn tires. Goodyear also conducted wet surface tests on surfaces with .02 inch of standing water, which is more representative of typical wet road driving conditions.
24 Docket No. NHTSA-2000-8572-26.
25 The Aerospace Corporation, Evaluation of Techniques for Reducing In-use Automotive Fuel Consumption, June 1978.
26 Docket No. NHTSA-2000-8572-26.