19.2.1. The slow‑moving vehicles signs, diagram 583 (S2‑2‑49, see Figure 19-2) and the associated plate “Slow lorries for” a distance (S2‑2‑49, see Figure 19-3), are intended for use on roads where the gradient is such as to have an adverse effect on the speed of heavy commercial vehicles, thereby causing a potential hazard to other vehicles in the same lane. This is not simply a question of congestion. The problem can occur even when there is more than one lane. At times of light traffic flow, a vehicle being driven in the left hand lane at 70 mph might close unexpectedly rapidly on a slow‑moving lorry, or on a queue of vehicles waiting to pass it.