IV. TARGET POPULATION

Safety Problems associated with Low Tire Pressure

There is no direct evidence in NHTSA's crash files 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. 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 is more likely to cause loss of control or a skid initially. Skids can lead to tripping and then a rollover.

The 1977 Indiana Tri-level study associated low tire pressure with loss of control, on both wet and dry pavements. They never identified it as a "definite" cause of any crash, but did identify it as a "probable" cause of the crash in 1.4% of the 2258 crash investigations. ⁽¹⁾ Note that more than one "probable cause" could be assigned to a crash. However, 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 1977 results may not be applicable in today's tire environment.

Low tire pressure probably causes crashes indirectly. Such tires wear prematurely and unevenly, making them more vulnerable to belt failure, punctures and skidding. 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. We will only be able to identify these indirect crashes after we can associate pre-crash tire pressures with crash types.

The target population for general tire-related caused crashes

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 that 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 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. 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).

The FARS data were further examined to determine whether heat is a factor in tire problems (see Table IV-2). 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.

1. ¹ 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.