No, I mean having a gap between separate groups of tiles, which are actually one city in practice. The gaps would explicitly NOT be filled with tiles, but instead with map features created earlier, such as rivers, a terraced height differential, etc.
You could also place an ordinary raised expressway or boulevard prefab in between tiles (150m is just too wide for anything other than some 6+ lane monster expressway), or even just have one of the splat-painted roads running through town, if you want to keep it low-effort and don't mind the lack of detail.
The point is that you could take tiles which would ordinarily be laid down at exact 150m increments and separate them (or, manually place them in groups arranged that way) allowing for the introduction of features which the tile system does not current accommodate well, such as rivers, larger urban thoroughfares or expressways, odd-shaped park spaces, areas to create differences in elevation (moraines etc.), and so on.