Does the capacity of the road influence how many agents can travel down it simultaneously?
No. Number of lanes and whether there are stop signs or traffic lights only affect traffic agents. They do not affect non-traffic agents.
- Traffic agents include police, fire, ambulance, garbage trucks, delivery trucks, moving trucks and of course sims driving their cars.
- Non-traffic agents include power, water, sewage and of course pedestrian sims.
I don't have a good explanation for why your sewage was behaving the way it did, but the behavior described does match the behavior of sewage when there is a sewage deficit.
Normally, sewage processing buildings pulse out a pull to sewage, and the sewage moves one way.
When there is a deficit, the building will pulse out a pull until it has absorbed the amount of sewage it can for that pull, at which point it will switch to a push! Sewage agents will move toward and then away, usually making no progress, and generally accumulating at their current position.
I solved my sewage deficit by plopping a sewage treatment plant. It pulls in the equivalent of 5 basic sewage structures at 4 times the hourly cost - and it solves the ground pollution problem as well.
Power and water agents are absorbed as they pass buildings. While they aren't affected by road capacity, they are affected by passing many buildings. If you layout your city with a powerplant at position 1, it is unlikely that agents will reach buildings at position 2.
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Update: There was a sewage issue fixed in patch 1.8.
Fixed a clog in the pipes where sewage would sometimes get backed up
and act jittery.