What is the shape of a "Keep Right" sign, and how should the driver react when he seesone?

Actually the shape isn't the concern as much as the coloring. Regulatory signs are black/white, or white/red, black/white/red. Regulatory signs tell you what the law is - they regulate you. They are notmally rectangular.

Warning signs are yello or yellow/green and are normally diamonds (except for school crossings). Warning signs just give you a warning of conditions, such as hills, curves, schools and pedestrinas, etc.

Constructions signs are orange/white and come in different shapes.

So, if you see an orange Keep Right sign, it is for construction. If it is black/white, that is the law.

In either case, by the time you get to the sign, you should be on the right!

you usually see these signs when you come to a median or a fork in the road...the sign means to stay to the right, not move into other lanes because you now have traffic in an opposite direction in the other lane