black black on white direction law no alternative one way one way signs only sign signs specific street signs white arrow black-on-white communicating communication directional gray grey indication no-alternative one-way one-way-signs pointer pointing reflective street-sign traffic traffic-flow traffic-sign arrows blue divergent diverging splitting splitting-up turn turning two-way alert alternative-way announcement be-careful beware blocked blocked-path care careful caution danger detour directing discoloration go-around informing lookout message observant observe pedestrians safety security warning watch watch-out wording words yellow left-turn vehicles vehicular