TIGER fixup/Divided roads

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The original TIGER data sometimes have a single way for each road.

Interstates, and other major roads, are often "divided roads" — called "dual carriageways" in Europe — for example, a northbound carriageway and a southbound one.

This guide uses Potlatch, the online editor, and requires some basic familiarity with adding points and drawing ways.


(to come)

Step 1: Tidying the first carriageway

Make sure you have the aerial imagery turned on. If you can't see it, check the "options" box.

Select the existing way, and zoom in for a good look. Move the nodes on it until it lines up with one of the carriageways.

Step 2: Creating the second carriageway

Select the whole way, then choose "Parallel way" from the "Advanced" menu (bottom left).

There are several presets for the distance between carriageways. However, these are for European roads where the carriageways are closer together, so you may need to add your own distance.

  • For a divided road in a city centre, select "Motorway".
  • For a divided road in suburbs or elsewhere, select "Choose offset (metres)" and type 25 into the box.

Click "Ok" and two parallel ways will be created in red, one on either side. If it's not right, delete the ways (select them and press Shift-Delete) and try again with a different offset.

When you are satisfied with the offset, delete the new red way you do not want, leaving the one for your new carriageway.

Click the padlock by the way id (bottom left) to unlock it. It will no longer be red.

Step 3: Tagging the carriageways

Copy tags from the first way to your new way, by using the "Repeat tags" button (bottom right).

Add a oneway=yes tag to each way. Make sure that the arrows are pointing in the direction of traffic. If they're not, select the way, and click the direction icon (bottom left) to reverse them.

Step 4: Beginning and end

Go to the beginning of the way, and make sure that your new carriageway is connected to the road beyond it, so that routing programs know there's a connection. If you haven't done the next bit yet, don't worry; just add a temporary connection. You can fix it when you return to do the next bit.

Do the same at the end of the way.

Step 5a: Fixing end-on junctions

You're almost done! All you need to do now is fix the junctions along the way.

End-on junctions are easy to fix. You will often find these on National and State Highways, especially in cities. You won't find them on Interstates.

If the junction stops at the first carriageway, check the aerial imagery to see if the road should connect to both. (You can press Caps Lock to dim the roads for a better view.) If it should, just select the joining road, and extend it to the other carriageway.

If it already crosses one carriageway on its way to another, you'll have to make a junction between them. Junction nodes are indicated by a black outline, so you can spot them easily.

When there's no node there already, just select the crossing road, and Shift-click over the carriageway. A new junction node will be added to both ways.

If there's a node there, but it's not joined, then select it and press "J" (for join). The node will be added to the other ways that cross at this point, and you should see the black outline appear. (You might have to adjust the node position first.)

Step 5b: Fixing grade-separated junctions

A grade-separated junction is one with ramps (slip-roads) and flyovers, so that traffic on the main line is never obstructed. Grade-separated junctions are more work, but they look great when they're done.

You will often find that the ramps are there in the TIGER data already. These are usually approximate and you'll find it quickest to delete them.

Trace the ramps from the aerial imagery, branching off the main carriageway. Make sure it links with the crossing road with a junction node.

Tag these as links. Choose the most important road in the junction — let's say it's an Interstate (highway=motorway). That means its ramps should be highway=motorway_link. (In other words, the "motorway"-type regulations also apply to the slip-road.)

Check that the crossing roads aren't mistakenly connected to the main carriageways. If there are junction nodes where they shouldn't be, just select them and press "-" to remove the node from one of the ways. (There are more details in the Over-connectedness HOWTO.)

Does the crossing road have a bridge, or does it go underneath the divided road? Choose the one that goes over the top, and by selecting nodes and using the scissors tool (bottom left), snip it so that the bridges are separate ways. Then select these ways, and add a bridge=yes and a layer=1 tag.

Finally, check that every ramp is one-way in the correct direction. If not, add a oneway=yes tag and click the direction arrow until the arrows are right.

Come back to the map in a day's time to see your handiwork! A properly-done grade-separated junction can look great.