Most people don't drive speed limits. They drive the speed the road is designed for. When you put arbitrarily slow limits on highways most speed. Those that don't then create dangerous speed deltas and road rage. These well known behaviors are already included and designed into the stats and engineering.

Typically highways will have higher percentage of fatal incidents but a much much much lower frequency and lower absolute number per mile. Well designed variable rate highways, even with a higher % fatality rate overall are safer.

NHTSA Road Type Crash Rate (per 100M VMT) Fatality Rate (per 100M VMT)

Urban Local Roads ~350–500+ ~1.5–2.0 Rural Local Roads ~200–300 ~2.0–3.0 Urban Collectors ~200–300 ~1.0–1.5 Urban Arterials ~100–200 ~1.2–1.8 Rural Arterials ~80–150 ~1.5–2.5 Urban Freeways/Interstates ~60–100 ~0.5–1.0 Rural Interstates ~40–80 ~0.6–1.3

By pushing traffic more quickly and reducing congestion you'll lower the totals while increasing the frequency that an individual is killed.

The speed must be designed into the infrastructure and if you're designing roads where you commonly have a 20 mph delta it's likely a shitty designed urban artery. And unless you're going to invest in trains and start kicking people off the road or put them into debt for speeding, people are going to do what makes sense at the time.