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).
Determine which type of tire tread depth gauge you are using, U.S. tread depth is establish in 32nds of an inch, other countries measure tread depth in millimeters (32/32nds or 25-26mm) Please use a tread depth gauge, follow the steps below, STEP 1. Confirm which measuring scale you are using. Some tread depth gauges measure in 32nds of an inch (left), others measure in millimeters. STEP 2. Confirm that the tire tread depth gage is accurate by pushing it down on a hard, flat surface.