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).
Use a tread depth indicator or gauge. You can use a special tread depth indicator or gauge tool to measure your tire's tread. If you don't already own one, the gauge is cheap to purchase from an auto parts dealer and it's easy to use. You may be able to find a downloadable tread depth gauge by searching online.