Which trail encounters would be necessary to share with another trail user?

1. Grizzly bear ahead
2. Bridge is out
3. Horses on trail
4. Rattlesnake on the trail
5. The route to obscure petroglyphs 
6. Band of sheep with guard dogs

All of these might be essential news to share, and none of these could be newsworthy. The proximity or impact of any one of these items to the point at which we encounter the other trail user creates context. 

If the encounter is imminent, then the warning is highly valuable. However, the snake I encountered over an hour ago has likely departed. The Grizzly bear seen yesterday may be in a different drainage. The horses headed back to the barn are no longer relevant to the other trail user’s experience. If the obstacle will change the route for the other trail user, then proximity is less important. If the only bridge crossing a major river is impassable then a warning one hundred miles in advance is relevant.

There is the human dynamic, with so much stimulus in an outdoor experience it is easy to forget about warnings. The information shared at the trailhead might be overlooked by the time I reach the area of impact. The level of severity and impact intensifies the classification of the information. A band of sheep chased by a grizzly bear trying to outrun a wildfire may stay with me longer: the more disruptive the potential interaction, the more relevant the trail report.