And as for unmarked police cars...
Yes, there are some unscrupulous individuals out there who could produce a fake badge and install some flashing lights in their car, but...
As with any crime, It's only a crime if you're caught. So...
Speed limits - whether they are followed or not - are posted for a reason, and they are the law. Ideally, we should all be driving the speed limit whether or not there is a patrol car within our sight.
The other night was just so "funny" because there were people pulled over to the side of road every mile or so. And they were pulled over by unmarked police cars that had their flashers on. If you were driving along and did not see all the police activity, well... You have no one to blame but yourself.
*And I remember unmarked police cars starting to be used when I was in high school. In fact, the police even released press releases to warn drivers to expect it when they least expect it. Usually, they would take a junker and equip it with a new engine. Nowadays, they're equipping later model cars - I've even seen a Lexus police car - for patrol duty.