Converting a divided highway to a single stroke in Illustrator

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Because there is no obvious way to merge two parallel lines into a single line in Adobe Illustrator, I needed to come up with a way to do this that doesn’t involve redrawing lots of lines with the pen tool.

So, here’s the process for converting two parallel lines into a single line.

  1. If needed, duplicate the layer so you preserve your original data.
  2. Make the parallel lines black.
  3. Merge the parallel lines graphically be increasing the stroke weight.  This makes them look like a single fat line.
  4. Rasterize the line using Object->Rasterize.  I don’t think the resolution should affect things much, but I go with 150 ppi.
  5. Use Image trace -> Line art option.
  6. Expand and style the line.

The result is a single line.

What to watch out for

I tried to automate this by creating an action.  The result ended up with the line being outlined instead of resulting in a single line.  I don’t know why the Image trace selection wasn’t preserved by the Action.  As a result, I just do this manually.

MAPublisher notes

Because I import the GIS data into Illustrator using MAPublisher, it retains attributes (like route number) that I may want to use.  I usually move the original data to the bottom of the layers palette  (below the base layer of my map) and query this data even though I don’t use it for presentation.