Demonstrably untrue. There is an entire very technical industry for this sort of thing. People who take an interest in their communities and go to planning and city council meetings know this. Those who are not involved their communities just complain and blame it on The Man out to get them.

You need to break pretty hard to not get fined.

The industry being technical does nothing to prevent it's general rules from being dictated by politics. If an engineer does a study that shows no significant increase in injury by increasing the speed limit 10 MPH, I find it highly unlikely the city or state is going to approve it based on those findings.

My morning commute involves a blind left turn out of a residential street. Cars parked on the curb obstruct my view of oncoming traffic in the right lane. This isn't a problem when that traffic observes the 35 mph speed limit. I've had far too many close calls with self-centered jerks driving 55 mph in that lane. Sporty sedans are impossible to see over a row of parked cars.

Therefore, the fact that it's left up to the drivers to guess when it is safe and when it isn't means many folks are going to guess wrong and speed when it's unsafe. Would be much better if the signage just actually accurately reflected the safe limits.

Driving within a reasonable margin above the limit is safe on most roads, under good surface and visibility conditions.