20/20 vision is defined as 1 arc minute of resolving power. It is rare for anyone to achieve resolving power more than twice that.
1 arc minute translates to 87 dpi at 1 meter, although I have no idea why you mix inches and meters here. It is 95 dpi at 3 feet; 100 dpi is the commonly used number. People with 20/10 vision can resolve 190 dpi at 3 feet, 175 dpi at 1 meter.
No one living sees better than 300 dpi at 1 meter, so it is not likely to be the standard in ANY country, much less "most". 600 dpi for road sign legibility is even more absurd.
At 1km, 20/20 vision can resolve a "dot" about 29cm in size. That's 3.5 dots per METER. 1/2 meter letters would not be legible. 20/40 vision, a common driving standard, would be closer to 2 dots per meter, or the feature size you are quoting.
A road sign that should be legible at 1km should have a minimum letter size of 1.1 meters, not 0.5 meters.
2 dots per meter at 1km is 2 dots per mm, 50 dpi, at 1m not 600 dpi. In order to resolve text at that size someone would need 250 dpi of acuity which no one has.
Carewolf, everything you said was wrong. You may need a new calculator.