The sign has that design because it used to mean the end of the speed limit - you'll also hear it referred to as "deristricted" although that hasn't been accura

The sign has that design because it used to mean the end of the speed limit - you'll also hear it referred to as "deristricted" although that hasn't been accurate for many years. Rather than spend a vast amount of money replacing the signs and potentially also missing some and resulting in driver confusion, the National Speed Limit was introduced.