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).
For decades, a quick test for tire treadwear has consisted of placing a penny on edge in a tireâs grooves. If the top of Abe Lincolnâs head remains visible, the tread is less than 2/32nds of an inch, meaning it was time for new tires. In fact, tread depth below 2/32nds makes those tires âillegalâ in some states. Even without a state law, most all industry players acknowledge the danger of riding on the worn-out tires.