Signage
In most of the world speed limit signs display the limitation within a red circle. This design follows the style set out by the Vienna Convention on Road Signs and Signals. The United States and Canada do not do this, while Botswana uses a blue background. All countries (except the United Kingdom and United States) use metric speed limits in kilometres per hour. Metric signs are optional in the United States though rarely seen, while Samoa uses both units.
Read more about this topic: Speed Limit