Thank you Captain Obvious of the roads department for that insight into the physics of rocks. They actually do stop, once they reach the bottom of the cliff or the top of your car, whichever comes first.
I’m being silly, of course. The two signs said falling rocks (warning) and do not stop (advice/direction).
Never Pure and Rarely Simple 
Many signs one sees on a road trip can be interpreted as exhortations (obeying which is generally not recommended), notably “END CONSTRUCTION” and “WASTE PAPER.”
Jokes about signs warning “SLOW … CHILDREN” became so widespread that I’m sure such sign wordage must have been completely phased out.
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Somewhere on the internet is a photo of someone standing next to a sign saying ‘Draw bridge’, holding a sketch pad in their hand.
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There are lots more examples. I read some time ago about a contest a couple of guys had on a road trip, obeying the signs (some of which were for local businesses). The prizewinner responded to a sign (presumably at an auto body shop or supplies store) that read “Clutch Parts.”
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In some (?most) languages this kind of ambiguity is impossible, because nouns and verbs are clearly marked as such.
I once read a joke about a little old lady who saw a sign saying ‘CAST IRON SINKS’ at a hardware shop.
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On a recent trip I saw BORE WATER (tell it about your tax audit). On our most recent trip I saw FALLING ROCKS again, and thought that it probably doesn’t (you may have to think about that one!). I also didn’t see SLOW BUSES – if you’re the driver, apply the brakes, if you’re a pedestrian, hold on tight. (It actually said WATCH FOR SLOW BUSES and is presumably for pedestrians).
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