Deceleration is a word rarely seen on road signs. Acceleration, the rate of change of velocity, is a vector. That means it can have any direction. Since it is usually measured along a line like a road, it is given a positive value if the velocity is increasing or a negative value if the velocity is decreasing. This negative value for acceleration is called deceleration. Clearly if you were waiting in the lane next to the sign, you would have no rate of change of velocity so would not be decelerating and would have no business in that lane.
