No parking with a double horizontal-arrow plate

The figure shows no-parking sign A and supplementary plate B with two opposing horizontal arrows. Plate B does not indicate directions of travel: it shows that the restriction extends along the road on both sides of the point where the sign is installed. It is false to say that adding plate B makes the restriction permanently effective. The arrows define its spatial extent; its duration and any time limits depend on the main sign, other plates and the applicable rules. Read this family of plates as boundaries of the main sign's validity: a single arrow identifies one side or a start/end point, while opposing arrows indicate continuation or validity in both directions from the sign.