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 tread depth groove in the tyres will tell you when your tyres are at the bare legal minimum of 1.6mm, ideally you should change your tyres earlier. The best way to check you tread depth is to buy a tread depth gauge.The gauge will give you the exact amount of tread remaining and will a lot more accurate.ost tyres now also have a depth indicator as shown in the picture above. The depth indicator is placed around the tyre within the groove. This is an easy way to check your tread depth. Once this groove indicator levels off with the top the tread the tyres are at 1.6mm.