A couple of different ideas with the styling applied to the tunnel part

Dashed across

::casing {
  line-dasharray: 8,8;
}
::fill {
  line-dasharray: 8,8;
  line-cap: butt;
}

Dashed across, casing around dashes

::casing {
  line-dasharray: 9,6,1,0;
}
::fill {
  line-dasharray: 8,8;
  line-cap: butt;
}

Inner dashes

::casing {
  // no changes
}
::fill {
  b/line-color: mix(white,@motorway-inner, 60%);
  b/line-width: @motorway-z14-width * (1 - 2*@motorway-casing-width);
  b/line-dasharray: 8,8;
  b/line-cap: butt;
}

Dashed casing

::casing {
  line-dasharray: 8,8;
}
::fill {
  // no changes
}

Alternating dash casing

::casing {
  b/line-dasharray: 8,8;
  b/line-width: @motorway-z14-width;
  b/line-color: mix(@motorway-outer,@land,20%);
  b/line-cap: butt;
}
::fill {
  // no changes
}

Existing

This is a dashed casing implementation with high frequency dashes and a fill colour alteration

OpenStreetMap Carto

This is a dashed casing implementation with moderate frequency dashes and an obvious fill colour alteration

None of these options are difficult to implement. Dashed across and dashed casing require the fewest lines of code, I prefer alternating dash casing and inner dashes.

I find that the dashed casing options look a bit strange with width changes.

Of course, the size of the dashes can be tweaked.

@Pnorman in order of (my) preference:

1. Dashed casing
2. Alternating dash casing
3. Dashed across, casing around dashes

Dashed casing implemented in https://github.com/kartotherian/brighmed/commit/fb445ef85cda3add39d69a35cdbd90eff07a4f94