I think the worst thing any commuter can do is ride a route without checking it first. I was almost guilty of this. I have been tossing around the idea to ride my bicycle to work for quite some time. It seemed to be slowly coming to a reality as I morphed into more of a commuter. I changed my mountain bike into more of a pavement bike when got the rack, road tires, panniers, lights and installed my computer on it to track overall mileage. The computer is mainly for keeping track of parts wear and tear than anything.
Well I mapped out a route via Google maps. I decided to drive the route, even though the drive to and from work would be a bit longer. The added time is caused by the lower speeds. I am glad I did. One intersection was quite congested and would not be very bike friendly. Quite frankly, it was down right dangerous. I am not easily convinced that an intersection is dangerous. However, this one had several cars and I have seen some crazy people driving stupidly because they are in a rush to get to work. I do not want to be around that.
So I started looking at other options. I was staring at Google intensely when I saw a road. I did not consider this road because you cannot physically access it easily by car, but by bike it is very easy to access. The issue is that one road dead ends, but there is only about 20-feet of dirt to ride through, and you are on the other road. Due to the fact that it was a dead end, and I could not access it with my car, I totally overlooked it. Come to find out not only is that road less congested, it is a quicker route to my work!
So, unless you have traveled that route several times in the past, take the time to drive it a few days in a row. This will help you get a good feel for the traffic patterns.