That's the speed LIMIT, not the suggestion, nor the MINIMUM. Limit means "maximum." While it is common for people to drive 5 over, it's still illegal. And by definition, illegal is unreasonable, or in other words "negligence per se." Also, the laws everywhere demand that you drive according to weather, lighting, terrain, etc. When it's dark, it's smart to slow down due to less visibility, inherent difficulty seeing, and other notions. "Brighter lights" is not going to match ideal daylight conditions, nor is it the solution. The solution is to slow down a bit.
These concepts are all on your drivers ed test. If you disagree, try answering that it's okay to drive 5 over, and okay to overdrive your headlights at night, on your next test, and see how it plays out, explain to them that it's reasonable to speed just a little and overdrive the conditions.
On a 2 hour drive, following the law by slowing down by 10 mph from 80 to 70mph means you arrive ~17 minutes later. But you increase your reaction times and stopping abilities significantly, increase accident survivability by a large factor, all contributing to the difference between living and dying for everyone involved. Plenty of studies show that high speed is a major cause of an accident, and also an exponential factor in vehicle accident fatalities. This study shows that nearly all impacts over 60mph are fatal, and the fatality rate goes to 100% over about 70mph.
https://www.littlerock.gov/media/2484/the-relation-between-speed-and-crashes.pdf
I'm not here to lecture people, but I have noticed that folks on the roads these days drive like they're getaway drivers on a bank heist.