People don't leave home planning to crash yet on some highways only a few oddballs keep below the posted speed. Since crashes are rare, posted speeds should be corrected to use the wisdom of highway planners (and the wisdom of drivers). They advise posting the speed at which 85% of the drivers would choose to drive that road if there were no speed limit. It's just as dangerous to have a slow driver in the fast lane as it is to have a speeding driver dodging in and out of lanes. Roads are safer when speeds are similar for all the cars on them. I drive below the posted speed on congested roads, when pedestrians are about, when the road is damp or icy, and when it is dark or the road is windy. I drive above the posted speed when traffic is light, when there is a passing lane, when no people or creatures are walking near the road, whenever I can see clearly ahead more than the distance I need to brake to a complete stop. I like to get home safely but without wasting time.
So, congratulations, BC, on adjusting some speeds upwards.
Politicians are faced with interest groups who like slow speeds which drivers generally disregard because they use their own judgement about what's safe. (Really, what chance does a child have of avoiding injury when run over by a car at 10 kmph compared to being run over at 30 or 50?)
A lot of revenue is created from speeding tickets. One Missouri town of 50 people has a police force (full and part-time) of 8 and generates most of the municipal budget from speed traps.
Remember when OPEC held the world hostage briefly and speed limits across North America were mandated lower to save fuel. (Possibly 1% was saved). That's another reason most speed limits are lower than they should be.
Posted speeds are helpful to assess road conditions ahead that a driver may not know about. Making them absolute criminalizes the population that wants to use its own judgement to drive safely. Making them absolute degrades the individual and exalts the state.
Road signs should be a source of useful information.
Summarized from Alex Mayasi's interesting and longer article at Priceonomics.