I saw the sign...

This last weekend I went on a road trip with my sister to southern California. On the 9 1/2 hour drive I kept seeing those yellow signs that say, "watch for falling rocks"...and I just had to wonder what the point of those signs actually is. Because if you are watching for falling rocks you aren't looking at the road as you should be..but a more realistic question I asked myself is, even if I do see a rock falling...is there really anything I can do about? At 70 mph I really am moving too quickly to avoid any potential damage from the falling rock. Or is it supposed to be one of those crazy word problems..."if the rock leaves point A on the mountain traveling as x speed and I see the rock at point B at my current speed of y, at what point will the rock irreversibly damage my windshield or kill me on the spot?" I mean really, those signs are pointless...the only use I see for them is for the state to avoid a law suit in the unlikely event that a rock does in fact fall from the mountain...then they can say, "we told you to watch for falling rocks even though there was nothing you could have done except to anticipate your demise" but still I just laugh when I see the sign