I am trying to understand why the viewshed analysis is taking so long (nearly 124 hours) to run. I have a DEM raster file for all of Colorado which has almost 3400 features and a polyline national forest roads file that has nearly 9000 features with various segment lengths such as 0.002 to 41.5 miles. After the last 124 hour running it completed but no raster data can be seen. The values vary from 0 to 255 and I am unable to view the attribute table. What am I doing wrong?
Viewshed is slow because it is complex. It will first convert your polylines to raster, so a very long line will converted into a large number of pixels (so one 41.5 miles of road ~= 750 pixels on a 90 DEM). Then each "observer" pixel is taken into account to check if it is visible from any point of your DEM. This is simply huge.
If you want to gain som speed, you could use a outer_radius value that will reduce the searching distance. If you have several computers, you could also do the work in parallel and combine all results in a sum of pixel values. But there is nothing magic to run a complex task.
Concerning your raster, it seems (to be verified) that the sum was limited to 8-bit, and you are asking for far more than 8 bit. I think that viewshed2 has a limit of 32 bits, so this should be enough for you.