Highway speed limits are significantly less than conditions allow but speeding is a huge problem on residential streets. Over the last 40's the highways have as a matter of policy been engineered to be pretty safe for people going a bit over the speed limit. This conditions drivers to speed and then they carry over this behavior to streets where is generally not even safe to go near the speed limit most times of day.
Really we need to revisit the whole approach to speed limits. We can make them electronic on all highways and adjust them to conditions. Then we can set them at 90 mph when conditions warrant, but in exchange we should enforce these speed limits with an iron fist. If you go 1% over you get a $50 ticket, 5% $500 ticket, 10% over $5000, 15% $50,000, etc. with various Jail times for greater speeding. And forget having cops for this, just cameras that take a picture of the perp. We can have cameras every 1000ft and we can forget about conspiracies to signal the presence of speed traps.