I'm not sure if this a bug, user error, or just the way the roadblock functions.

It seems that when housing is separated from other buildings by a roadblock the max distance for employees is reduced.
To clarify, when there are no houses near any building requiring workers, it may say "poor access to employees" or "no employees live nearby".
My problem is that when I place a road block to keep services contained within a residential block that distance that buildings can get workers seems to be drastically reduced SOME of the time.

My city had about 15% unemployment so plenty of workers. The buildings in question were 6 road squares away from the housing block. The 5th road square had the roadblock in it. I set the game speed to 100% and watched them just to be sure and over the course of 3 years they only had employees for about 6 of the 36 months.

Removing the roadblock had all the buildings (6 brewers and 3 warehouses) fully staffed almost instantly but then of course all the services are spilling out of the housing block.

I've encountered this issue before in other situations where buildings that should have been close to receive employees simply didn't until I removed the road block.

Is there some mechanic I'm missing?

I having a hard time seeing it. Is there a visual you could put up?

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"North" of the Green and "west" of the red circle there is a warehouse. Leave the road block but try putting a road that allows the walker to continue the square-- a road that allows a walker to walk past the roadblock and then continue north along Beer Row. The warehouse is gone and it isn't Euclidean. But it will let the walker to continue without removing the road block by the beer producer at the intersection. Also, by the weapons factories, there is a road. Would that be a detour that is keeping he walkers from co-operating. One final solution. You have a Bazaar and Temple two blocks from the roadway. There is a police station almost at the roadway. If you remove these structures and move housing within one square of the roadway, the walker will pick up workers. Because there is one vacant square along the walkway (for landscaping), then the new housing, it takes longer for the Walker to get the necessary workers. But the stretch between the temple and the police station should be long enough. I hope I am being clear. Walkway. Open Space, Housing. It would require substantial re-working of the Red circle area. But you would have housing between the Police Station and the weapons area from which walkers would pick up workers. (There is another roadway by the Temple I can't quite see that may make this a problem.) But if the by-pass for the road block doesn't work, I think you will need to move some housing closer to the road that is "west" of the red circle.
Again I hope this is clear. If not let me know.

Just saw the second pix. You may have to save. Destroy one of the structures, rebuild, and then follow the walker to see if that area has too many choices for a walker. Blocking one long section of road is ugly, but I've done it.

Edit for some spelling mistakes