standard city streets, plus about one-half mile dedicated bike path, on each of two separate routes. One I follow getting to work, and the other returning home. The ride is more interesting that way.
Started commuting in 1970, while living in Tucson, AZ. There my round trip varied from 14 to 22 miles round trip. One of the major reasons I have continued with bike commuting is that I find that I arrive at work more alert and moe functional, in just a much better frame of mind, than I do when I drive. Fortunately, my employer, the National Park Service, has become increasingly more supportive of bike commuting. I now have a shower and a locker at work - commute valhalla!
I think it is important to consider the time at which you commute as well as the route. I prefer to come to work early, while all the cars are parked. Parked cars selcom cause problems for a cyclist. Getting home is different. There is plenty of traffic, so I make sure I return home during daylight hours, or else, call my wife for a ride (I also call for a ride in really stormy weather). During the winter, this means I take an hour of comp time off, so I can return while the sun is up. Thank heaven for flex time.