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).
Checking Tread Depth. Most tires in production today have little raised rubber bars built into their design, called tread wear indicators (TWI), that show you when your tread depth reaches the recommended replacement threshold (usually around 1.6mm [1/16th inch] for street tires, off-road tires are more subjective).