U.S. Route 66 was decommissioned in 1985. If any route relation purports to represent that route, it’s probably lying, because the 1985 alignment is no longer contiguous due to highway realignments.
OpenHistoricalMap is the best place to reconstruct all the old alignments. So far, we’ve mapped the route around its endpoints in Los Angeles and Chicago and through much of Arizona and Oklahoma. But almost none of it is in route relations yet. We’d welcome any help in completing this route, given its importance to the U.S. in the 20th century.
Any route relation tagged network=US:US:Historic represents Historic Route 66, which is part of the America’s Byways program. Participating states post a shield that resembles the old U.S. 66 shield, but it isn’t a real U.S. Route shield. Depending on the state, this route may precisely follow the old alignment, sometimes giving you a choice of era, or it may only vaguely follow the old route along modern highways. These relations go wherever the signs go.
Ideally, any route that crosses state lines should have a route relation for each state, joined by a route superrelation, since it’s technically multiple routes that happen to share the same number. That’s even more apparent from the diversity of signage above.
When AASHTO decommissioned U.S. 66, some states maintained the old route number as a state route. In Oklahoma, State Highway 66 and Historic Route 66 are largely concurrent:
This is not at all uncommon: often, when a state decommissions one of its own routes, it reverts to a county route by the same number, as a last-ditch attempt to salvage some prestige and economic benefit for the communities along the route.
Like the U.S. Numbered Highway System, the U.S. Bicycle Route System assigns even numbers to east–west routes, increasing from north to south. This means bike routes and highway routes of the same number can meet or overlap.
There are three relations in Oklahoma because routes with signposted cardinal directions should ideally have unidirectional route relations.
Aside from these examples, be on the lookout for other route relations that don’t correspond to anything on the ground. Over the years, roadgeeks have attempted to maintain relations in OSM that represent private tours approximating the old route as closely as possible, but on modern streets according to some criteria. These route relations are misleading at best. If they’re still around, they should be deleted.