Having driven in the US and UK, this is a significant difference between the two. In the UK, you might sometimes drive 30 under on a road that is nominally 60 mph. In the US, that road would have a specific posted speed limit that is safe to drive. US roads are also more consistently designed for constant speed or have additional advisory speed limits for curves. You can nearly always drive as fast as the number on the sign unless there is some additional hazard.