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).
The two horizontal lines in the lower half of the plot represent two criteria for specifying the lifetime of a tire. The lower curve represents 2/32, the legal limit in most states for minimum acceptable tread depth and the height of tread wear indicators built into the tire. This depth corresponds to the common Lincoln head penny test. The second line represents 4/32, a depth that may represent a safer minimum. The first plot shows the actual measurements of the tread thickness. I've had the tires rotated twice and there's an obvious change in the wear rate at each rotation.