The large diamond shaped sign shows a black arrow on a yellow background telling drivers of a curve ahead. Underneath it is a smaller square sign with black lettering on a yellow background showing a speed of 30 km/h. This sign and others similar to it are classed as advisory signs by the Motor Vehicle Act Regulations.
The signs are advance notice of conditions on or adjacent to a highway that are potentially hazardous to traffic. A driver may choose whether or not to follow the suggestion given by the sign. Ignoring the advice is not an offence in itself, but anything that happens because the signs are not given consideration may be an offence.
Advisory signs generally have black figures on a yellow background.
The example of the curve was chosen to illustrate a point. We have often seen these signs and then travelled around the curve comfortably at speeds higher than that suggested. In those cases the shape of the curve and the road condition could accommodate the vehicle travelling at the higher speed.
So why was the speed warning there? Often it is because the driver's line of sight is restricted. This would prevent the driver from seeing and reacting to a hazard in or just beyond the corner unless the speed was at or less than that suggested. Heavy trucks may also be required to slow for the corner to prevent tipping over.
Many considerations must be made in order to drive safely. As you can see, some are not immediately evident. Reference Links