Today’s tutorial comes from an anonymous question that I received in my inbox, which asked:

I design a bus transit map using a street layer. But how can i align correctly multiple lines on a street without overlap?

This is a great question. You’d be amazed how often I see people attempting to draw multiple parallel route lines manually, which is absolutely the most difficult way of doing things. You might be able to get away with it on a rectilinear transit map, but it’s almost impossible to get right when your route lines are overlaid on a standard road map – there are always curves and twists in the road that make it nigh-on impossible to manually draw adjacent route lines without things looking terrible.

For an example of a very poor attempt at drawing parallel route lines individually, you need look no further than the light rail map for Denver, Colorado (April 2013, 2 stars).

The most frustrating thing about seeing it done the wrong way is that there’s a tool in Adobe Illustrator that makes this task almost effortless.

Object Menu > Path > Offset Path…

The images above demonstrate how to offset route lines correctly – the first image shows how it’s done with an odd number of route lines; the second illustrates how to do it for an even number of route lines.

If you’ve drawn the street layer on your map yourself, then you’ve probably already got the first thing you need – a path that follows the centre of your street. Copy it and then Paste in Front (Cmd/Ctrl-F) so it sits exactly on top of the original. Then move it to its own layer above the street layer and stroke it the way you want your route lines to look. In my example, we’ve got an 8-point wide red route line. 

If you’re drawing your route lines on top of an aerial photo or Google Maps image, then you’re going to have to draw your own central route line. Be as accurate as possible while also using as few bezier points as you can. Again, put it on its own layer and make it look the way you want. Now the fun starts.

IF YOU NEED THREE ADJACENT ROUTE LINES (First image), then you’re going to use the central route line you already have, and use the Offset Path function to create a new route line on both sides at once. Select the path, then invoke the Offset Path command. In the resultant dialog box, enter the amount of offset you want, which is the desired distance between the paths. Here, I want a little gap to show between my 8-point route lines, so I entered 10 points. The middle part of the first image shows the results: Illustrator will offset your path to both sides of the line, so it’s basically done all the hard work for us. Now you just need to use the Scissor tool © to cut and then delete the pesky little joining lines that are created at the top and bottom (circled in blue in the image). From there, simply colour the route lines as required. If you need five, or seven, or more route lines, simply keep offsetting the outermost route lines by 10 points and then cutting the resulting new lines to only keep the outermost section each time.

IF YOU NEED TWO ADJACENT ROUTE LINES (Second Image), the process is almost the same, except that when you select and offset the original path, you only use half the required distance between route lines (in my example, 5 points). Then you can delete the original route line, keeping only the new offset paths, which now sit neatly on either side of the centre of the road. To create four, or six, or more route lines, continue to offset the outermost paths by 10 points as in the first example.