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.