[ryantgtg]

In California (and I believe many other states), what you describe would be considered a speed trap, and it isn't allowed. Speed limits are supposed to be set by measuring how fast people drive on the roads. If 85% of drivers go 40, then that's the speed limit. If a city/county hasn't surveyed a street recently, then an officer cannot park their vehicle and hand out tickets for speeding (though they can give a ticket if they are driving their vehicle and see someone speeding) - because that could be a speed trap.

In 2016, the speed limit was enforceable on only 19% of streets in Los Angeles due to the speed trap law. The city has made great efforts recently to update their speed surveys, resulting, in most cases, in an increase in speed limits.

[amanaplanacanal]

The roads are engineered wrong. If you design a road for 40, then set the speed limit at 25, it's not going to work. You have to re-design the road for the 25 MPH speed limit. There are several ways to do this, and they are well known.

Of course cities are chronically underfunded for infrastructure, so this is really hard to do in practice.