Monday was the first business day that commuters headed over the new Tappan Zee Bridge.

Four lanes of Rockland-bound traffic started flowing across the first phase of the Gov. Mario M. Cuomo Bridge early Saturday morning after four years and $3.9 billion in construction.

The Monday evening commute on the bridge had traffic flowing at a steady but slow place.

Many signs alerted drivers to stay in their lanes because of the new traffic patterns on the span.

The commute on the new bridge will be a lot quieter because the old construction metal plate on the previous Tappan Zee Bridge made for a noisy and bumpy ride.

Construction of the bridge began in late 2013. Officials say Westchester-bound traffic will move onto the new bridge this fall, and the entire project will be fully complete in 2018.