How is the Speed Limit sign something you need to pay attention to and have highlighted in red?
First of all most people drive down the same roads regularly and know the speed limits, this system would pretty soon become irritating and a distraction.
I don't believe that most people remember the speed limits on the roads they drive regularly, and even if they do, speed limits can change. Leaving it to the general populous to assume their knowledge of local speed limits is infallible doesn't sound good to me. I agree that constant reminders of what you are already aware of would be irritating though, like a girlfriend who keeps pointing out that all of your clothes are crap...
The Speed limit for a road is pretty much set by the type of road it is, you really don't need to see a sign to have a good idea of the appropriate speed for a particular road .
Again, I disagree; I live near two towns, one with a 15 MPH speed limit (no, I'm not kidding) on their main street, one with a 40 MPH speed limit on their main street. As with the above, I'm not comfortable assuming the speed limit, so I'm sure as hell not comfortable with you or anyone else assuming it either.
Finally the GPS which your almost certain to have, if you have this system, knows how fast you are travelling and the speed limit for the road.
Red is pretty much always used to indicate danger of something critical it's a bad color to use for that sort of information amber might make more sense if you have to highlight this sort of information. I would save red for things in your path or moving into your path - real dangers.
Speed limits are not setting the speed for a road the road conditions are and the driver needs to be able to evaluate the road conditions and drive at an appropriate speed.
If a driver is incapable of determining appropriate speed and evaluating the road conditions then they really shouldn't be driving.
Not everyone has a GPS unit, I know only two or three people who have and use GPS devices. I have one myself and I can assure you that it gets the speed limit wrong plenty of times, in an out of towns. Sometimes it thinks the speed limit is higher, sometimes it thinks the speed limit is lower, and I've updated it recently...
The bottom line is that people need to take responsibility for the way they drive, but they're not going to. Automatic transmissions, cars that park themselves, traction control, cars that automatically brake if it thinks we're getting too close... all have their good points (except automatic transmissions, man up and learn to drive stick already) but all move us closer to a time when the machines will be doing practically everything for us. The disconnect between driver and the world they are traveling through is widening; I think most people no longer really consider the potential consequences of poor choices/failure to pay attention when driving. The more cars end up doing for us, the more time that leaves us to text, apply make-up, shave, etc... A frightening thought.