 The purpose of the blister surface is to provide a warning to vision-impaired people who would otherwise, in the absence of a change of height of >25mm, find it difficult differentiate between where the footway ends and the carriageway begins.The surface is therefore an essential safety feature for this group of road users at pedestrian crossing points where the footway is flush to the carriageway, to enable wheelchair users to cross unimpeded.The profile of the blister tactile surface consists of rows of flat-topped blisters in a square pattern.