I think what this does, though I stand to be corrected, is adjust the estimates in Basecamp (you can do similar in the preferences menu in Mapsource). These settings do not transfer to the GPS when you upload the route.
Basically you're stuck with the calculation that it makes across the given road types. Not sure how difficult it would be for Garmin to add a slider to the menus to make this adjustable, not that they're going to bother... I would imagine that there's a factor in there somewhere that takes the total distance and calculates a time. Can't imagine that it assesses every single road in your route, the length of each leg and the speed limit applicable. It's always going to be an ETA (stress on the E = Estimated).
Pretty easy to have an adjustment figure based on experience and apply that yourself, that's what I tend to do. Just adding 15 minutes to an hour depending on distance. road type etc. With motorway/autoroute I can pretty well nail the ETA, over more varied roads I factor in a little extra time.
I think we're a little spoiled with all this technology and expect it to do too much for us...
