TheoTown

When building standalone cities, I notice that vehicles do not use neighbour connections (ie they do not travel to "neighbouring cities").

but commercial and industrial building can travel to another neighbouring cities

Even if they're not in a region, I feel the system can pretend there are neighbouring areas, and allow vehicles to drive outside.

if standalone cities are single forever, i will make a tips & tricks for doing that

You can move a copy to a region if you like.