Commentary: Legal Minimum Tread Depth for Passenger Car Tires in the U.S.A.—A Survey

Abstract

Available tire traction is a significant highway safety issue, particularly on wet roads. Tire-roadway friction on dry, clean roads is essentially independent of tread depth, and depends primarily on roadway surface texture. However, tire-wet-roadway friction, both for longitudinal braking and lateral cornering forces, depends on several variables, most importantly on water depth, speed and tire tread depth, and the roadway surface texture. The car owner-operator has control over speed and tire condition, but not on water depth or road surface texture. Minimum tire tread depth is legislated throughout most of the United States and Europe. Speed reduction for wet road conditions is not.A survey of state requirements for legal minimum tread depth for passenger vehicle tires in the United States is presented. Most states require a minimum of 2/32 of an inch (approximately 1.6 mm) of tread, but two require less, some have no requirements, and some defer to the federal criterion for commercial vehicle safety inspections. The requirement of 2/32 of an inch is consistent with the height of the tread-wear bars built in to passenger car tires sold in the United States, but the rationale for that requirement, or other existing requirements, is not clear. Recent research indicates that a minimum tread depth of 2/32 of an inch does not prevent significant loss of friction at highway speeds, even for minimally wet roadways. The research suggests that tires with less than 4/32 of an inch tread depth may lose approximately 50 percent of available friction in those circumstances, even before hydroplaning occurs. It is concluded that the present requirements for minimum passenger car tire tread depth are not based upon rational safety considerations, and that an increase in the minimum tread depth requirements would have a beneficial effect on highway safety.

Regulation and the Risk of Inaction

“…Prominent examples come from Nevada and California, the two states whose departments of motor vehicles were directed to quickly enact regulations governing automated vehicles and automated driving. 2 These regulations seek both to provide greater legal certainty to the developers of automated systems [16], [26], [31] and to restrict unreasonably dangerous products and practices [23].…”

“…Speed is not the only relevant driver action. Tire condition, for example, is an important consideration in stopping distance, is at least nominally regulated [2], and yet varies widely within the current vehicle fleet. If the hardware on automated vehicles is expected to be regularly inspected, so too should the hardware on conventional vehicles.…”

Regulation and the Risk of Inaction

“…Tire condition, for example, is an important consideration in stopping distance, is at least nominally regulated [31], and yet varies widely within the current vehicle fleet. If the hardware on automated vehicles is expected to be regularly inspected, so too should the hardware on conventional vehicles.…”