I'd like to add some perspective to this thread...
From a historical standpoint, "trails" on American public land came about through three different processes:
- Historical trails. These were existing wagon roads, mining trails, and livestock driveways that have been adapted as "trails" in the modern era.
- Fire access trails. Most of these were built in a huge burst of activity in the 1930s through the CCC. One interesting point about them is that the men (and they were all men) who laid them out were all working from the same plan.
- Recreational trails. Most of these were built post WWII, since before then the idea of going hiking in the woods for fun was somewhat odd and only a few eccentrics and high rollers actually participated in such activities. Also during this time some trails were built for motorbikes and OHVs.
The important thing to note is that only a tiny minority of these trails were ever built with bicycles in mind, and the vast majority of those trails, for reason of tread width, sight lines, and vertical and horizontal clearances are at best marginally safe and at worst insanely unsafe for cyclists or to be shared with cyclists and other trail users.
Since a high-water point in the 1970's, there has been a long and inexorable decline in the number of trails and miles of trail available. This during a time period when the population in the United States very nearly doubled. As a point of reference, where I live on the East slope of the Cascades, eighty to ninety percent of the trails that existed during the 1970's have simply ceased to exist.
It gets complicated because different land management agencies have dealt with the benign neglect in different ways. The Park Service has gotten creative and managed to keep most of their trails open. A lot of that is because, as I pointed out in a different thread, each Park has a lot of flexibility in how they accomplish their mission (one book I read described each Park as an independent fiefdom, and that description is not far off) and that let them get creative. The US Forest Service and the BLM, on the other hand, gave much less flexibility to local administrators and also had other missions and priorities. The net effect is that the vast majority of trails (probably on the order of ninety percent) administered by these agencies have ceased to exist.
Now take that perspective to a hypothetical hiker or horseman who lives in the Seattle area. The number of trails accessible to her has substantially decreased, by 75 percent or more, in the last few decades. Now they are being told that they have to share what is left of those trails with a new class of trail users. On trails that were never designed for those kinds of users in the first place.