I recently drove I-84 southbound at night and found the posted 80 mph speed limit to be simply insane.
There are deer and elk crossing warning signs posted all along the Snowville area, where 80 mph speed limits rule.
In the day, that might be fine, but at dark, that's a disaster waiting to happen -- especially when you factor in a lot of curves and mountain grades.
I think 65 or 70 mph at night is more reasonable for that stretch of road, until it meets I-15.
-The same is true for I-15 north Malad, Idaho. In those mountains, an 80 mph speed limit at night is unwise too.