TR21 Ice Grit Signs, also known as a TW-4.1 or a W3-2.1 sign, are used when grit is spread onto the roadway surface when ice forms. Where several sections of road occur in close proximity, the first sign may be augmented with an additional TW-1.1 Next "" "" km T121 temporary warning (TW) supplementary sign. Ice/ Grit Signs can be used on Level 1 or 2/3 Roads.