Stroke - dashed line with gap that increases then decreases

Alexa,

You can create the appearance with symmetrically increasing, constant, and decreasing, gaps in one go as follows, the exact details depending on the desired range of gaps (how small to start and end with compared with the largest constant gap), and which proportion to have the constant gaps):

1) Create a vertical stroked path with a length/height equalling the apparent Stroke Weight of the apparent dashed line, and with a Stroke Weight equalling the desired apparent dash length;

2) Move a copy of 1) horizontally a suitable distance to the right, the exact distance is unimportant but should something like the total length to be covered;

3) Object>Blend>Blend Options, set Orientation to Align to Page and set Spacing to the total number of apparent dashes less two;

4) Object>Blend>Make, then Ctrl/Cmd+C+F, then hide the original;

1) - 4) should give you a basic Blend and a copy to fall back on;

5) Create a circle with a 1pt Stroke and a diameter that roughly corresponds to the desired combined length of the end parts with increasing and decreasing gaps, then Ctrl/Cmd+C+F, then hide the original;

6) ClickDrag with the Line (Segment) Tool horizontally across the top half of the circle, crossing each side at a suitable height, possibly corresponding to 30 degrees, depending on the desired start gaps: the smaller gaps, the lower;

7) With the horizontal path still selected, Object>Path>Divide Objects Below, then select the lower part and delete it, then Direct Select the straight bottom segment and delete;

8) With the Direct Selection Tool click the top Anchor Point with the Scissors Tool, then Alt/OptionClick the right part and ClickDrag it horizontally to the right by a distance corresponding to the desired length of the middle part with constant gaps, then Direct Select both top Anchor Points and Ctrl/Cmd+J, then Ctrl/Cmd+C+F, then hide the original;

5) - 8) should give you a basic Spine to create the right Blend and a copy to fall back on;

9) Select the copy Blend from 4) and the copy path from 8) and Object>Blend>Replace Spine; this gives a first feel of the dash/gap appearance, just ignore the different vertical positions;

10) Object>Blend>Expand, then Ctrl/Cmd+Shift+G to Ungroup, then Align>Vertical Align Center, then Ctrl/Cmd+G to Group;

9) - 10) should give you the fundamental dash/gap appearance; you may redo as far back as you wish, until you are satisfied, maybe especially to 6) to reuse the circle from 5) and adjust the height of the dividing line;

11) Select the Group in 10) and click New Brush in the Brushes palette, then select Art Brush;

12) Apply the Art Brush to a path with a 1pt Stroke Weight that represents the birds-eye view of the bridge.

Some of the steps shown: