Regarding a (at least in the US) I think a big part of the problem is a system of laws that people generally agree is unjust (i.e. the drug laws)
Regarding speeding, I don't see how the current system can hold up under technological advances. There are a dozen trivial ways to tell if someone is speeding, from the toll booth data of the original article to cell phone GPS to computer vision to road sensors to smart cars. It doesn't make sense for the police to perform inefficient, dangerous traffic stops when all this data is available, but the system would collapse if everyone received 10 traffic tickets a day.
I agree with your sentiment. But please bear in mind, that the profit per radar gun would probably go down, if enforcement was less sporadic.
But it does provide a way to pick-and-choose when enforcing laws.
That said, while people may complain about particular speed limits, I don't think very many would call a cap somewhere below 120mph "unjust".
Plus there's the issue of driver's ability as well. Can you look in your side mirror and successfully gauge the speed of the car that's a good distance behind you to know whether you can safely change lanes? If the car is going at a known max speed, of which you are probably aware of as you are likely at that speed yourself, then you can change but if the car is going two to three times faster than you I would imagine changing lanes would be a bad idea.
Also, how would the police be able to judge the things you describe looking at your car driving by at 120 MPH while everyone else is driving in the 80s? The officer would have to constantly be pulling these people over to check their "licensed to speed" status. How would the officer be able to judge the conditions of your car as you breeze by? Sticker on the bumper? Better be a huge freaking sticker when you are flying through traffic.
Speed limits are decided upon on the safety of most cars for the general area and expectations of traffic conditions, not the capabilities of the cars or drivers on the road.
However, there is one common case where the speeding law seems unnecessary at best. I live in Texas. Florida is similar in that there can be large stretches of empty highway. When I travel outside the city there is often very little traffic and very open country. If I can see for a mile and I am the only one out there, why shouldn't I travel at the fastest speed I can manage safely? In the city speeding rarely pays off, what with traffic and traffic controls. On a trip of 200 miles (Austin to Dallas for example) the difference between an average speed of 60 m/hr and 90 m/hr is an hour of travel. It seems wrong that I should be limited in this situation because an 18-wheeler cannot handle more than 60 or 70 safely. Worse is when you have small towns that drop the speed limit to 55 or 45 for a tiny stretch of road when the road does not even go through the town. That kind of speed trap is what angers people, especially when you see an officer blow through with impunity.
http://cbsdallas.files.wordpress.com/2010/10/i-35-texas_6713...
(Ok, that's downtown Austin; how about this one:
There are examples of US interstates with no speed limits within, to some people, reasonable limits:
http://www.motorists.org/press/montana-no-speed-limit-safety...
They even discuss how in some cases fatalities go up with new road laws and speed limits.
I have driven that stretch of road between Austin and Dallas as I live in the Dallas area. I have also lived in Florida with it's stretches of long, boring roads. Plus, I've lived in Vegas with that incredibly boring route into Los Angeles. In every case I would agree with you that the idea of being able to go a bit faster would have been nice.
But the question that will come up is how do you determine the maximum speed that you can safely manage? What if your assessment of your max speed differs with that of the police officer who witnesses what he feels is a dangerous amount of speed? What about other people on the road that you will eventually come across that can't handle the situation that you present if you decide that the person can handle a speed that in truth they cannot? I'm sure you're thinking you'll just slow down when you see someone but you cannot foresee the possibilities plus your reaction time dwindles quite effectively the faster you go. A mile at 60 MPH is one thing, a mile at 120 MPH is another.
My example against this, which is interesting considering the topic of this thread, is when I was in Vegas an off-duty cop was speeding (I forget how much but in excess of 100 MPH) on one of the interstates outside of the main part of town and rear-ended a family in their car. The resulting accident killed a number of them including children. Therefore, I personally don't like the idea of a free-for-all on the interstate where different cars can drive at different speeds based on criteria that will be different for each individual driver and car. I would rather we all agree on a maximum speed that makes sense for the conditions of the road and expected traffic. Now if a road has too slow of a speed limit for the typical conditions of that road, then that is a different discussion.