Is there a commonly understood signal that is used to warn riders of a road hazard?

As guardrail says, the standard slow down signal is about all you can do.

It's common practice to point at a hazard with a finger.....but it doesn't work so well when the moose is around the bend.

I have heard it's practice to point with a foot in some places....but they ride on the other side of the road.

Really, about all you can do is tell them to slow down and then hope they know what they're doing on a cycle.

Most riders look for a courtesy wave from an on-coming rider.

The universal sign of "slow down" is pump your left hand up & down

with you palm facing down.

That's about all you can do.

Yes there are hand signals. Here's a link:

http://www.mwsbike.com/hand_signal

We point with our foot off the peg (on whatever side the hazard is on) as we maneuver around it. Hands are for turning...

You could flash your lights. That's a pretty universal signal.

The other one is if there is a police. Try honking or sticking your hand out. Haha idk

NO