I drive about 50% of the time and bicycle the other 50% when the weather allows, and there are days when I have to decide how I get to work based on what the weather is, what I need to do when I'm at work/after work etc.... There have been many days when my careful planning on bike commuting has changed at the last minute, and enough of them end up being days when it's better to drive than ride that I don't worry about it anymore and I just go with the flow. I'll make up the riding somehow.