My second trip was to travel from this same friend's house in Springfield to the Whole Foods in Vienna. Unlike the first trip above, in which I was familiar with the first part of the route (and could adjust based on my knowledge), in this case I was completely dependent on the Google directions. I had never ridden in this area before.
In this case, the directions proved to be excellent, with one exception. The implementation of the directions presented some problems, but the actual route worked almost perfectly--taking me along mostly back streets and some trail connectors right to my destination. The major obstacles in this case were Route 50 and I-66. The crossing at Route 50 actually turned out to be better than expected, because there was a trail connector that Google Maps was unaware of that I noticed when I reached the intersection of Pickett Road and Arlington Blvd. I-66 was crossed on Vaden Drive, a back street that crosses the Interstate with no interchange.
The biggest problem presented by Google biking directions is that trail signage is typically so poor--or not designed the way street signage is--that it presents challenges for giving and following directions. At every single intersection of two streets, there is a street sign. It was simple to follow the directions: Turn right at XXYZ St. or left at ABBC St. At each of these points, I was virtually 100% confident that I was following the directions as indicated. However, each time I had to travel on a trail, I lost confidence, because the trails did not have signs like the streets. On this trip, even the trails with signs had different names than what Google indicated them as. The most disconcerting part was when I was traveling along a trail and would encounter an intersection or fork in the trail. Google does its best, but without any signage, it just states "Turn left toward Vaden Dr." or the like. If the cyclists doesn't know where the named street is, then there is no way of knowing what those directions mean. Thankfully it was a sunny day, so I could at least tell what direction I was going and could correlate that with the line on my map.
Google biking directions is a good tool for cyclists. If jurisdictions would now please put up signage that will help cyclists navigate those directions, it would make the tool work much better.