Speed limits aren't always black and white
Dear Dr. Gridlock:
I hope you might explain the perennially perplexing issue of speed limits, which universally never at all seem to be limits (per that definition).
In a rational world, the limit would be the uppermost allowable speed for any stretch of road. And to exceed that limit by even 1 mph would be a breach of law. This would be easy to understand and easy to teach to the general public.
Instead, nearly every domestic jurisdiction allows a leeway of up to 10 mph over the limit, beyond which one is then considered to have breached the law.
The only reasoning that I can cipher from this law enforcement curiosity is that it allows those in law enforcement to pull anyone over, for suspicion of any kind, should the driver's speed exceed the limit by even 1 mph. Essentially this enforcement habit lulls the general public into a degree of vulnerability, that law enforcement officials can then use to their advantage when they want to stop a suspect for investigation but otherwise have no legal justification to do so.
DG: Driving 1 mph over the speed limit is a breach of the law. In the Washington region, a police officer can stop a driver any time for driving faster than the speed limit.
Clearly, that's not how most officers go about enforcing the speed laws. They use some discretion. But all the driver should need to know is what the speed limit sign says.