Animate a string of arrows

The easiest way to animate a string of graphics (arrows) is to move them along a path until the second arrow gets to the same position as the first arrow. When the distance moved is perfect, set an outpoint at that keyframe for the layer, Pre-compose, move all attributes to the new comp, and trim to the layer length. Then, apply Time Remapping to the nested pre-comp. This will give you a keyframe at the first frame and one at the end of the last frame. If you move to the last keyframe, the screen will go blank because the keyframe is after the layer ends.

Move back one from the last Time Remapping Keyframe, add a new keyframe, and then delete the last keyframe. You should now see arrows at the first and last keyframes.

The last step is to add a simple loopOut() expression to time remapping. It will now appear that all arrows are constantly moving. 

You can stack the arrows individually or put them all on the same shape layer by simply copying and pasting. You can also use the Echo effect to generate a string of moving arrows. If you have already created an arrow that moves along a curved path, just duplicate the layer and offset the timing enough to create your evenly-spaced arrows. Pre-compose, then use the move exactly one position trick to set up your loop.