Because speed limits are quantized: 30, 50, 60, 80, 100, 120. The 50->30 (30->20 mph) is a well known baseline 0 change for built up areas. It's meant to be no

Because speed limits are quantized: 30, 50, 60, 80, 100, 120. The 50->30 (30->20 mph) is a well known baseline 0 change for built up areas. It's meant to be not for major roads, it's meant for neighborhoods and other areas with many pedestrians.