This little tip is thanks to RG, who left a comment on the site asking:

“Can you comment on how you make the 2pt of white space between lines show when you have lines cross over each other?”

On most transit maps, route lines will cross over each other at various points. Most of the time, an interchange station exists at that point and the symbol for that covers up the lines as they cross. However, sometimes the route lines will cross without any interaction between them – maybe there’s a bridge, or the lines are in separate tunnels – and it’s a good idea to visually separate them to make that as clear as possible.

STEP 1 above shows the set up: the Red and Blue lines cross over the Yellow and Green lines. As in the last tutorial, these are 8-point-thick lines set on a 10-point grid: I’ll use this a lot, just because it makes the maths easier to follow.

As you can see, the resulting 2pt gap between the Red and Blue lines allows the Yellow and Green lines to butt right up to them and show through the gap – neither of which is desirable. The quick and dirty approach to this problem might be to copy the Red and Blue lines, Paste in Back (Cmd/Ctrl-B), change the stroke colour to white and make these new strokes thicker. This does work, but it’s not the best approach in my eyes. It makes moving or editing those paths further down the track a two-step process (move the coloured route lines, then move the white lines that were underneath them), or a very fiddly single step: selecting objects behind other ones can be a real pain.

So I like to take advantage of a little-used feature of Adobe Illustrator – the fact that one path can have multiple strokes applied to it.

In STEP ONE, I’ve selected the Blue Line and made the Appearance palette visible (Window menu > Appearance or Shift-F6). To add a second stroke to the existing Blue one, simply click and drag that stroke down to the “Duplicate Selected Item” icon at he bottom of the palette as indicated. You can also just click on the stroke, then click on the icon, but I find it more satisfying to drag for some reason. If you really want, you can use the little flyout menu at the top right of the palette and choose “Add New Stroke” or “Duplicate Item” while you have the stroke selected in the main palette.

Bingo! The path now has two strokes, both of which are identical. Fortunately, the palette gives you everything you need to change the new stroke, as shown in STEP TWO. Use the little colour selector to change the second stroke in the palette (which is the stroke that’s behind the other one: the palette displays the stacking order of the path’s elements) to white, then change its stroke width.

The maximum amount you can increase the stroke by is twice the gap between your route lines – in this case, that’s four points (twice two), for a maximum width of 12 points. Any more than this, and the white stroke would start overlapping the Red line next to it. Normally, this amount is exactly what we’re after, so it’s not really a concern.

STEP THREE simply shows the result of performing the same steps on the Red line. Done! The advantage of this technique is that both strokes – being on the same path – can be moved or edited at the same time: there’s no need to fiddle around with two separate paths, one on top of the other.

And if you’re really smart, you’ll make Graphic Styles of these double-stroked lines, so that you can apply them again in the future with just one click.