The road is now mostly owned and maintained by the state through the Oregon Department of Transportation as the Historic Columbia River Highway No. 100 (still partially marked as U.S. Route 30; see Oregon highways and routes) or the Oregon Parks and Recreation Department as the Historic Columbia River Highway State Trail.
Measuring Tire Tread Depth with a Tire Gauge. While U.S. coins can be used to estimate remaining tread depth as a tire approaches the end of its useful life, the established method of measuring remaining tread depth in the U.S. is with a gauge that reads in 32nds of an inch (other countries measure tread depth in millimeters).
Here in the U.S., tire tread is measured in 32nds of an inch. A brand-new tire has a tread depth around 10/32nds of an inch (approximately 1/3 of an inch). Some tires, like winter tires or off-road tires, may have a deeper tread. As you drive on the tires, the tread gradually wears down. If you reach the point where thereâs 2/32nds of an inch of tread or less left, the tires are bald and are considered legally worn out in most states.