Starting Sunday morning, westbound traffic is now being shifted to two eastbound lanes, creating an "S" curve.
This is so crews can replace westbound bridge decks.
You can see the lane realignment and construction zone on a Caltrans map.
Westbound vehicles now move across the median for about a quarter-mile before moving back to the westbound side just before the toll plaza.
Caltrans says westbound traffic will be slower than normal.
The western drive onramp on eastbound I-580 is also closed during construction, which could last up to three months.
Caltrans spokesperson Allyn Amsk says the speed limit in the area where crews are replacing the westbound bridge deck will be reduced to 45 mph.
A detour will be in place on an existing bicycle road that runs underneath the highway.
(Bay City News contributed to this report)
