Attitudes like yours are what embolden governments to engage in revenue enforcement the proliferation of which leads to people tipping each other off about said revenue enforcement (e.g. flashing their lights to warn of speed traps). People wouldn't be warning each other about the cops if "people the cops shake down" and "people who were doing dangerous things just prior to being shook down" had more overlap.
I'm not sure I understand your point, unless you're arguing that national speed limits are deliberately low so that police can fine people driving at the "correct" speed? Or that police lie about your speed and fine you anyway?
The former is definitely true. There are towns out there in the United States that are on an interstate and play games with their speed limits. You'll see a speed limit of 60-70 up until you hit city limits then it suddenly drops to 30-40, with enforcement lined up at the city limits waiting to nab people.
You normally would, but this is just predation on through traffic. It's not like you're going through the middle of main street, these may be off to the side of town with several exits but close enough for the city to dictate speed limits.
Flashing lights is meant to communicate to others they need to slow down or otherwise obey the traffic laws. How does that have a negative effect on safety?