Interstate Highways

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Editing Interstate Highways

Interstate highways imported from TIGER data originally needed a lot of fixing. It was often easier to erase a section of the interstate highway imported from TIGER data and retrace it over aerial imagery or GPS traces. Nearly all of this has now been fixed at the interstate level.

Here are some guidelines to consider:

  • Separate interstate directional carriageways into two ways. For example, an interstate running West and East will consist of two parallel ways (two parallel carriageways): one running West and the other running East.
  • Apply the following tags to every way making up the interstate (some exclusions do apply, see I-180 in Wyoming as an example):
    • highway=motorway (Sometimes highway=motorway_link should be used instead, especially if a ramp is being used while the Interstate designation is changing to a different highway depending on the geometry of said ramp and if there are exit numbers are being used to designate an exit. An example would be like I-76 & I-80 completely swapping highways along the Ohio Turnpike:
    • name=* only if it has a name that is different from the route number (for example I-95/Cross-Bronx Expressway).
    • ref=I 40 -- if more than one highway is co-located along the same way combine them using semicolons: ref=I 40;US 1 . See ref tag detail below for important information about the ordering of these refs.
    • oneway=yes -- Validate direction of travel arrows for the way.
    • lanes=2 -- Verify number of lanes; generally you should include auxiliary lanes that run between ramps but exclude acceleration/deceleration lanes that are relatively short.
    • If known: maxspeed=70 mph and minspeed=40 mph (if posted). Don't forget to add the " mph" units to the value - the default is km/h.
  • Apply a route relations to all ways, see relations detail below.
  • When the interstate runs over a bridge, split the way at both ends of the bridge and tag it with bridge=yes and the appropriate layer=* (usually 1, unless crossing another bridge).
  • When the interstate crosses another way, but the ways are not physically connected, disconnect / un-glue the nodes where they meet. Add a bridge to the top layer way as shown above.
  • When the interstate runs through a tunnel, add a way section with tunnel=yes and the appropriate layer=* (usually -1, unless running under another tunnel).
  • For entrance and exit ramps use the following set of tags:
    • highway=motorway_link
    • oneway=yes -- Verify that the direction of travel arrows is correct.
  • For tagging exits: (The actual detailed exit sign information is usually available in the Wikipedia entry for a particular interstate)
    • highway=motorway_junction on the node which joins the ramp to the freeway
      • ref=364 -- Reference in this case is the exit number
      • name=Mid-County -- Optional exit name. Most exits do not have names. For an example of a named exit: Pennsylvania Turnpike
    • destination=Gettysburg;Harrisburg and other destination:*=* -- On the way that branches off from the motorway_junction node.
    • See Exit_Info for detailed exit tagging.


Main article: Interstate Highways Relations

Route Relations are useful for Interstate highways because they share routes, and often carry other routes for short distances. Interstate routing relations do not apply to exits or Rest Stop / service areas. Do not create a relation without first checking the Interstate Highways Relations page to see if one is already defined. If you create an Interstate highway relation, be sure to list it on that page also.

ref tag

The ref=* tag for Interstate highways should have the route designations as follows:

  • "I {route designation} {modifier}", where:
    • {route designation} is the numeric ("95", "495") or numeric and alpha ("35W") designation of the route. Letters representing cardinal directions do not form part of the official route designation and should not be included; cardinal directions, if desired, should be indicated using "role=" or by using the direction tag in Interstate relations.
    • {modifier} is an optional route modifier such as "Alternate", "Business", "Bypass", "City", "Temp", "Toll, "Truck" (rarely applicable)

Concurrent numbering (or overlaps)

If there are multiple Interstate routes that share the same physical ways, each of the route designations should be in the way's ref tag. This is called double-banding, or sometimes triple-, quad-, or multi-banding, by state DOTs such as New Hampshire, Louisiana and Iowa. Other states such as California refer to this situation as concurrent numbering or concurrency. OSM mappers sometimes describe this situation as an "overlap".

Concurrent numbering is also common where an interstate carries co-located US or state routes. For example, various sections of I 80 in the mid-west and west are co-located with US 6. US 6 "jumps onto" I 80, and is co-located for many miles. Later US 6 may jump back off I 80 and once again exist in its own right. In some cases, milepoints along US 6 increment across the jump according to old US 6 mileages; in others, according to the interstate's mileage; or else not at all (meaning the milepoints simply restart where they left off, before the jump).

Listing order in ref tags

The listing order of Interstates in ref tags was considered insignificant by early mappers, but OSM users become more diverse, this is no longer the case. It is important that the first-posted shield on the highway (usually at the top of the sign) is first-referenced in the ways' OSM ref tags. Often this is the lowest numbered, highest functional classification route; but not always. For example, I 80 is the official designation of I 35 and I 80 around Des Moines, IA. I 80 determines the milepoints and exit numbers across the common section. For this reason, I 80 should be listed before I 35 in the ref tags of those ways.

Another example of this would be in North Carolina along I 77 & I 74. When I 77 enters North Carolina from Virgina, I 74 starts to be signed. While I 74 is the lower of the two numbers, the highway segment was built as part of I 77 from the start, and I 74 was only added to it in the 2000's. Thus, all milemarkers are based off of I 77's mileage in North Carolina. Of the two posted assemblies for this 'multiplex' (1 for each direction), I 77 shields are posted before the I 74 shield. Thus, I 77 should always be listed before I 74 in the ref tag for those ways.

Similarly, but more obviously, interstate refs should precede those of lower functional classifications. For example, I 184 (Boise connector) must be listed before US 20 and US 26 near downtown Boise, ID.