How would a LIDAR help with the lines on the road? At best it could see the guard rail or another car in its path and break to avoid a crash but this could very well take a car off the side of the road if there's no clear obstacle.
There's no RADAR/LIDAR system that would reasonably tell apart real from fake signs or lines (the lines being the real dangerous issue). They come in many shapes and sizes. A fake sign would also fool a human but overall a human driver has many other cues to base a decision on. At the very least someone trying to trick you would have to invest far more into the props used.
[0] Though people would have an additional layer of reasoning about if the speed limit was reasonable for a particular set of road. Just putting up a 90 MPH speed limit on a random residential road even with a convincing sign should get people to do 90 on it. (Granted computers already have databases of posted speed limits they could draw from too)