This is kind of a tangent to my normal tutorials, but I had a surprising number of requests for this after I published my McKinney Avenue Trolley map, so here goes!

The first thing to note is that this is not a 100-percent accurate representation of the interchange: this trolley map is not intended to be a road map or to be used to navigate freeways. I want to communicate the idea of an interchange stack and show general connections, but I’ve left some of the fiddly detail out. However, this technique stands up pretty well to any level of detail required: it’s just a matter of how patient you are.

Let’s run through this step by step:

START
This just shows my base layers before any of the complex stuff starts. At the very bottom, there would be a some sort of source image template for tracing elements from (which I’m not showing to improve clarity), then the background colour layer, then two road layers. In this part of Dallas, some parts of the highways are actually lower than the surface roads, so they come first (in a layer called “Hwy Low”), then a “Streets” layer for those surface roads.

A note here on colours. I actually define separate global colours for the different types of roads I need to show on a map, so that I can easily tell them apart while working. So, as shown here, there’s yellow for freeways/highways, white for surface roads and blue for freeway ramps. When I’m done, I can simply re-edit all these global colours to achieve the final look of the map (in this case, all roads end up being white). I also define a “road edge” colour for the stroke that separates the different level of roads (black in its working mode, the same colour as the background in the final piece).

STEP 1
I’ve added a layer for those parts of the highways that are elevated above the surface roads, called “Hwy Up”. Once I’ve drawn the paths for the roads, I’ve copied them and sent them to the back (a quick Cmd/Ctrl-C, Cmd/Ctrl-B combo) and changed their stroke from a 6pt “highway yellow” to an 8pt “road edge” black.

Here’s the first of my clever tricks for this type of work: the top yellow line has a rounded cap end, while the lower black line has a flat cap end (these are defined in the Strokes panel). If both of them had a flat cap end, then you can often see a tiny thin sliver of black extending past the end of the yellow, especially with PDF output. It’s tricky to see with this image, but I’ve circled a deliberate example of this in “Step 2” above. If both strokes have round ends, then the black stroke extends all the way around the yellow one, which we don’t want. Having a round cap above the flat cap hides it effectively every time.

STEPS 3 THROUGH 5
Now it’s just a matter of adding all the desired freeway ramps and overpasses, working your way up from lowest in elevation to the highest. You can do this all in one layer if you’re confident in Illustrator, or you can make separate layers for each level as you go up. I thinned my ramps down to 4pt wide, with a 6pt “road edge” stroke behind them. The outside edge of the ramp stroke needs to line up pretty precisely with the outside edge of the freeway stroke below it: this probably takes the most practice to get right on a consistent basis.

You’ll note in the images above that even though the “road edge” stroke should cross over and be visible above the highway that’s on a lower layer, it isn’t. This is my second little trick: I use an opacity mask on the strokes where required to hide parts of them from view.

If you haven’t used opacity masks in Illustrator before, then I highly encourage you to do some research and make them a part of your workflow. Basically, you draw a 100-percent black object that defines the area that you want to be masked, making sure it’s placed above the stroke in the stacking order. Then, select both the stroke and the mask object and click the “Make Mask” button in the Transparency palette (you might also have to uncheck the “Clip” check box to make things show up as you expect).

It’s certainly not as intuitive as masking in Photoshop, but it saves having to shuffle things around layers in an often futile attempt to get them behind some objects while still being in front of others. In the images above, I’ve shown the masks I’ve used for each stroke as a magenta box, just to give an idea of what’s required.

Note: You could also use the simpler Clipping Mask function to achieve the same end result, although here you have to draw a masking object that covers what you want to be visible after masking, not what you want hidden. I personally find it easier to draw a small object over the tiny part I want to hide, rather than a large object that encompasses the rest of the relevant stroke.

STEP 6
We could really call things done after finishing Step 5, but I wanted to give the freeway ramps and overpasses a little more dimensionality and depth. I do this by copying “road edge” strokes where they cross lower layers and then pasting them behind. I cut them using the scissors tool so that I’m only left with the pieces that are required, then I just nudge them a few points directly down the page to give the illusion of depth. These are shown in green in Step 6 above.

FINISH
Now it’s just a matter of redefining the working global colours for each element to achieve the final look. For this map, that means changing all the road elements to be white, and the “road edge” colour to match the background colour. Beautiful!

See also: all my tutorials related to transit map design.