This sign is an informative sign indicating that the road is expanding with a new lane beginning on the right-hand side.

The traffic rules regarding positioning state that: "Insofar as circumstances permit, vehicles shall be driven on the right-hand side of the road. On carriageways with two or more lanes in the same direction, the right-hand lane shall be used when traffic rules do not require or permit use of the left-hand lane(s)."

Normally, this is the lane you should use. There are of course exceptions when it is still permissible to use the other lanes, e.g. to ease congestion.