So trails disappear when at scale of 0.5 mi when detail is set to "normal" and appear at the next zoom increment where scale is 0.3 mi.
Trails disappear when scale is at 2mi when detail is set to "most" and appear at the next zoom level when scale is 1.2 mi.
That seems consistent with what I would expect for display up to zoom level 2.
My approach may be different than the general user, but I like being able to see a large geographical area (small scale map) and have the trails appear when I'm hunting for someplace to go hiking. It's painful to have to zoom in to a very small area and then hunt around to see if there are trails in the area. Admittedly, having trails turned on at level 3 will clutter up things, but it's a simple enough option to either turn down the level of detail, or simply deselect the trail map so they don't appear if I am simply looking for roads (just as I'll usually turn off the topo maps so I don't see all the topography).
I suspect this is subject to personal preference, but it's really handy to be able to see the distribution of trails at small scale.
I'm using the etrex HCX.
I've avoided TYP files thus far since I can usually define a line as something that shows up fairly well on the etrex (e.g. I usually use "international boundary" 0x001e for my trails as this shows up quite well). I find that the Montana Trails map available GPSfiledepo has a nice definition for trails that show up as red dashed lines, but I have no clue as to how to decompile their TYP file to see how they managed that nifty trick (although this was addressed in a recent forum discussion on gpsfiledepot by Coggin about redline trails appended below ). Perhaps I need to delve into that unless you elect to change the color (I'm not lazy, I'm just not very adept).
from Coggin at http://forums.gpsfiledepot.com/index.php/topic,392.0.html
The process requires one to redefine the TYP for Trails/Walkways 0x16
There are a couple of programs that will do this, the demo of MapWel will and is what I use: http://www.mapwel.eu/down.htm
Also genTYP: http://cypherman1.googlepages.com/genTYP
will do this.
Custom Type files are detailed in the cGPSmapper manual starting on page 72.
Here's an example of redefining a trail to a green color:
Xpm="32 1 4 1"
"= c #00FF00"
" c #000000"
"3 c #007F00"
"4 c #000000"
" ============ ============"