Like you, I know my routes well and know when I can put down the power and when the route can hide a danger. I have never crashed with another trail user, ever. And to that point, I don't know of any crashes at this location.
These signs have the same purpose as school zone signs on the roads, the signs explain the reason you need to drive with extra cause, not just 'slow down'. Or a curve sign that's warning you to slow because of the road conditions. Simply "slow down" isn't helpful and just a little annoying.
Three Rivers manages over 240 miles of bike trails. I live where 5 of these trails come together, so I ride most of these 240 miles. Oddly, this the only place I've seen these "safe speed" signs, so they stand out as unique. Like I said before, there are many other seniors facilities along these trails. Funny enough, slow down sign near the middle school would be better, dozens of kids spill on to the trail that runs past the school as classes lets out and they are not paying attention to anything ... ever. (I avoid riding this trail in the afternoon hours, distracted drivers are less dangerous
)
Rabbit hole, off topic, long story:
The heartburn is related to the builders of this seniors apartment forcing leases to be broken to accelerate the construction of this apartment (all about the $$$). One of the businesses was where I boarded my dogs, small locally owned business that I'd worked with for years. I had a trip planned and dogs booked, when the developers forced the boarding house to close, leaving me to scrambling to find another place for our dogs on Thanksgiving week. It took that boarding house over a year to find a new location and get back in business. That sucks, it's nothing to do with the people that live there, but was the start of the heartburn.
The next heartburn is it's own thread -
https://www.bikeforums.net/mountain-...rant-hate.html - see the pic in post 5, you can see the apartment in the background.
Lastly, I've been caring for a few elder women in my life during the last few years. To be clear, I am an advocate for the elder (as well as people biking). OTOH I have an issue with the way some are trying to force changes to long existing trails and parks to conform to their wants instead of adapting to the place they picked to live. This article is related, but not directly connected to this issue -
Progressive Boomers Are Making It Impossible For Cities To Fix The Housing Crisis
In summation, the signs are just ??? - not a cheer or a jeer, just kinda WTF? and yes, there's a lot of baggage that I bring along this assessment, I'm certain nearly no one else has even noticed the sign.