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).
Another way to check tread depth is to look at the treadwear indicator bar that's molded into your tires. The bars are located at the bottom of the tread grooves in several locations around the tire. When these bars become visibly flush with the adjacent ribs the tire has no more than 2/32 of tread remaining.