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safety February 04, 2016

How Tire Tread Depth Affects Stopping Distance

What is Tread Depth? Tread depth is a vertical measurement between the top of the tread rubber to the bottom of the tire’s deepest grooves. In the United States, tread depth is measured in 32nds of an inch. New tires on most new cars measure 10/32 inches to 11/32 inches.

Why Tread Depth is Important? It is imperative for vehicles to have tires with enough traction to translate braking power into stopping power. In many states, including Virginia, a vehicle’s tires will fail at State Inspections when they reach 2/32 inches of remaining tread depth; meaning they will be legally worn-out. U.S. law requires tires to have easy-to-see Tread Wear Indicator bars running from one side of their tread design to the other when the tread has worn down to the minimum legal limit. According to a recent study, while the National Highway Traffic Safety Administration (NHTSA) found that nearly 26 percent of the tires with tread depths of zero to 2/32 inch contributed to crashes, eight percent of the tires with 3/32 to 4/32-inch tread depth also created problems in crashes.

People often rely on a tire’s tread depth to determine its condition, but since the rubber compounds in a tire deteriorate with time, regardless of the depth of the tread, an older tire poses a safety hazard. Automakers and tire manufacturers recommend tire replacement, regardless of wear, between five to 10 years. This will help ensure safe transportation before the rubber deteriorates to the point of failure.

Because tires have the greatest effect on the way a vehicle handles and brakes, it is unwise to wait to replace tires.

Wet Roads Affect the Stopping Distance of a Vehicle. How? Water cannot be compressed, so a vehicle needs enough tread depth to allow rain to escape through the tire’s grooves. Tires have channels for water to flow through; channels are designed to direct water away from the tire so it can remain in contact with the road. The deeper the channel, the more water can move. Tires that are worn down have shallower channels resulting in a dramatic effect on the vehicle’s ability to stop on wet roads. The vehicle may hydroplane, or float on top of the water, losing traction. Without traction, the care will not respond to the steering wheel or the brakes, presenting serious safety concerns.

In the DC metro area, we experience wet roads during every season. Consider replacing tires when they reach approximately 4/32 inches of remaining tread depth. Sufficient tread reduces hydroplaning on snowy, slushy and wet roads, affects stopping distance and helps prevent accidents.

Bringing a vehicle to a complete halt takes time. While advances in brake and tire technology make it easier for vehicles to stop more quickly and safely, factors such as weather and road conditions, along with the condition of the tires greatly affect the actual stopping distance.
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