The length of road to which a traffic sign on a road (except a parking control sign) applies is worked out in the direction driven by a driver, or travelled by a pedestrian, on the road who faces the sign before passing it.
Example : Rule 21 (3) provides that a speed limit sign applies to the length of road beginning at the sign and ending at the nearest of a different speed limit sign, an end speed limit sign, or speed derestriction sign, and the end of the road. Only the nearest of those things, in the direction driven by a driver, or travelled by a pedestrian, who faces the sign before passing it, is relevant.
Note 1 :
"Parking control sign" and
"traffic sign" are defined in the Dictionary.
Note 2 : Rule 332 deals with parking control signs that display an arrow. These signs apply to lengths of road.