Right. The real speed limit in VA is close to 80 mph (though you can get pulled over for doing less, presumably if "driving while black" [1]). So why aren't there signs that read "SPEED LIMIT 80"? Why is it up to the officer's discretion to decide if today it's 80, 78, 75, etc.? (I know why: federal government, funding, etc.)

I am in no way against speed limits being enforced, I just don't like the idea of them being set by the officer's mood and zeal on any given day. I'd rather know what the speed limit really is rather than trying to guess it.

[1] https://en.wikipedia.org/wiki/Driving_While_Black

Federal funding is no longer tied to speed enforcement. Highway speed limits are set artificially low because it's convenient and profitable for the police. (Given that statistics do not reliably support the usual contention that higher speed limits are more dangerous, the burden of proof falls on those who would argue otherwise.)

Maybe you're not speeding, but rest assured, there's some other attribute of your driving or your car's condition that will allow law enforcement to pull you over if they want. This isn't a coincidence. For better or worse, the police will tell you that they catch a lot of genuine bad actors this way.

It's not that they want to be able to stop everyone... it's that they want to be able to stop anyone.

Near me in VA there is a highway with two signs quite close together. One says "Speed Limit 55" and the other says (I won't have the words exactly right), "Trucks slower than 55mph must stay in right lane." The combination of signs seems to acknowledge that most traffic is exceeding the speed limit.

I've seen this. You don't want some log truck lumbering along at 53 mph in the center or left lane. Also it would be very surprising to be hassled for going 60 mph here.

They can, and do, issue citations for not obeying road signs like this, and I am glad for it.

That's exactly the issue. You should not be surprised when you are hassled. Currently, you can be pulled over for doing 59 in a 55. As far as I can tell nobody does 55, unless it is in a 45. You shouldn't have to guess what is acceptable in each county or town. You should know what the real speed limit is.

Is there a 4 mph "forgiveness" in the law somewhere? I thought that technically it was an infraction to travel 56 in a 55, which is equally ridiculous as being ticketed for going 59 in the same spot.

I own a car that encourages the driver to go fast (think Autobahn speeds) with its power and ride comfort. I am still going slower than 60 in the 55 zones.