If they weren't before, 420-mile markers are now an endangered species. Few major highways traverse 420 miles or more within the borders of a single state. By my count, the removal and replacement of the two signs in Washington, one sign in Colorado and now one in Idaho means that there may only be 11 420-mile markers left in the United States.
Three of these are located in the state of Texas, not exactly a hotspot of pot culture. The remaining eight are sprinkled across the country, from Wilmington, N.C., to just outside of Gustine, Calif. Here's the complete list, culled mostly from this Google Answers thread. Am I missing any? Let me know.
Interstate 5, California
Interstate 10, Texas
Interstate 20, Texas
Interstate 35, Texas
Interstate 25, New Mexico
Interstate 40, Tennessee
Interstate 40, North Carolina
Interstate 70, Kansas
Interstate 75, Florida
Interstate 80, Nebraska
Interstate 90, Montana
Interstate 70, Colorado (replaced)
U.S. Highway 95, Idaho (replaced)
U.S. Highway 12, Washington (missing)
State Highway 20, Washington (replaced)
Of course, there are a number of other highway signs that have attained notoriety among marijuana smokers over the years. Who could forget the famous Shamokin/Pottsville sign on Interstate 81 in Pennsylvania? Or Wisconsin's Bong Recreation Area? Or the entire town of Weed, Calif.? Not to mention numerous state and county roads in the United States numbered 420.
And now with the advent of legalization in places like Colorado and Washington, a whole new generation of pot-related signage is coming to fruition.