Marking a numbered highway

Ideally a numbered highway has a well-defined route and two well-defined endpoints. In reality, things may not be so simple. I will attempt here to summarize the issues in determining how to mark a highway, and offer suggested resolutions. Most of my knowledge pertains to U.S. practices, so I don't know how useful the details will be in other countries.

Ideal case

In the ideal case, both ends of US 1 are at at-grade intersections with other roadways, and have clearly posted "end US 1" and "begin US 1" signs. Every turn along the route is clearly marked, and a sufficient number of reassurance markers are posted after major intersections so that the motorist can comfortably follow US 1 from beginning to end.

In reality, things may not be so simple. The end may be at a complicated intersection with multiple legs - not a problem for the driver who simply wants to get onto another highway at the end, but possibly a problem for us. Important signs may be missing. And routes may literally split, with two parallel roadways both being either consistently or inconsistently marked as the same route in the same direction.

Complicated ends

Anything more complicated than a simple intersection may present a problem. At a three-way motorway junction, the ending route separates into two ramps (motorway_links), each roughly equal in importance. Even on surface streets, signs may end before the route itself terminates, so as to not confuse drivers. There may even be an "end" sign posted before the split.

There are two ways to handle this:

1. Simply end the route at the final split, where the motorway partitions into motorway_links.
2. Choose one direction as the through route, hopefully not arbitrarily, and end the route where it merges with another or leaves the junction.

Missing signs

First, a note on defining a numbered highway. In the ideal case above, there is a clear path in each direction, marked for the convenience of drivers. But, in many places, numbering also corresponds to maintenance responsibility; for example, if the U.S. state of Florida maintains a road, it will give it a number, and it will not number roads that it does not maintain (though signs may be posted through gaps in state maintenance for continuity). There are cases where the numbers used internally do not match the posted numbers; in Florida, this includes higher-level systems (U.S. Routes and Interstates). For example, most of what is marked as only US 1 is internally State Road 5, but a small portion is State Road 805, with SR 5 following a parallel - and signed - alignment of its own.

Often, if signs are missing, one can use maps or other records produced by the highway authority to determine the official routing.

But what happens if a route is unsigned, or is so badly signed as to be essentially useless as a through route? It may depend on the reason for the lack of signage - was it a deliberate choice not to mark that route, was it caused by years of neglect, or is it simply a coincidence that so many signs on one route are missing? Since our primary purpose is to help people navigate, we should generally not show unsigned routes (or street names, etc.). But sometimes, for continuity purposes, it may be appropriate to label what should be, but isn't, marked, so the driver can follow the route using other information such as street names. As for jurisdictions that have utterly neglected their signage, especially where no records are kept as to how the routes are supposed to be marked (for example, when a county maintains all roads in unincorporated areas, but only a few major ones are signed as county routes, we can't tell where a route goes by who maintains the roads), one solution is to only label short pieces where the routes are signed.

Other complications

Sometimes a route simply ceases to exist, or is deliberately not signed (see above), but after some distance it begins again. For example, US 1 may split into US 1 Business and US 1 Bypass as it approaches a city, with neither route being just plain US 1. After the city the two routes rejoin to form US 1. Or a state may have made a decision not to mark an overlap, even at its ends, and treat the minor route as if it ends at one end and begins at the other. (Be careful: sometimes the missing link is important enough that, as with missing signs, it may make sense to label the overlap as both routes.) But generally this should be treated as though the route ends on either side of the gap, just like two separate routes that share a designation.

Conversely, a route may have multiple parallel alignments. (I'm not talking about a one-way pair, in which each direction has only one clear routing.) For example, a motorway may split into local and express lanes, both of which carry through traffic and are signed as part of the route. Or, rarely, more than one route through a congested city center may be signed identically.

Summary

If in doubt, the following order should be used to decide how a route goes:

1. Follow what signs on the route itself indicate.
2. If those are missing, go by what signs on other roadways indicate, assuming internal consistency among them.
3. If the above does not provide enough information, and an official routing is defined by the maintaining authority, go with that.
4. Otherwise, make a judgment call, possibly using the note=* tag to explain your decision.