It varies by jurisdiction.
70
The safest maximum speed for that road
It is unknown who invented the speed limit sign. However, the first maximum speed limit was the 10mph (16km/h) limit introduced in theUnited Kingdomin 1861.
No, a speed limit sign typically indicates the maximum speed allowed on a particular road or highway. It does not specify a minimum speed. However, some jurisdictions may have specific laws or rules regarding minimum speeds on certain roads for safety reasons.
When weather and road conditions permit it.
Yes. A speed limit sign is an example of a regulatory sign.
In the UK the speed limit comes into force as soon as you pass the speed limit sign. As such the distance is zero as you should have slowed down to the posted speed limit before passing the sign.
In the US, the maximum speed on an interstate highway is 70. Until recently, there were a couple of stretches of highway that were limitless.
When it is flashing the speed posted is the limit
A motorist is expected not to drive faster than the speed limit on the sign until the school has been passed.
Advisory speed sign
A posted speed limit sign indicates the maximum speed permitted on the roadway for daytime or nighttime. In the case of the example in the question, the permitted speed at all times would be 55 miles per hour.