There are a multiple caveats on the data quality, but since there aren't many alternatives available, the ACS is still a valuable source of information of bicycle use and transportation trends. Some of the limitations become clear when we look at the actual question asked:
How did this person usually get to work LAST WEEK? If this person usually used more than one method of of transportation during the trip, mark (X) the box of the one used for most of the distance.So someone who bikes to work on two days and takes her car on three would count as a car commuter. Similarly, a person who rides 5 miles to the commuter rail station, takes the train downtown, and then walks to their office, would count as a train commuter. The other, less obvious limitation, which is especially relevant for Madison, is that students' trip to their university are not considered commutes and therefore not counted. And of course, trips to work constitute only part of all trips made.
That said, let's look at the data, beginning at the state level.
Maybe unsurprisingly, at the state level the picture for bike commuting appears rather bleak. Between 2006 and 2013 the rate of bike commuters has been basically flat and far below one percent. However, one must keep in mind that there are only six states in the whole of the United States that have a mode share of one percent or more, and the national average is 0.62%. Nonetheless, the flat trend raises some uncomfortable questions for bike advocates.
similarly high rate of bike commuting, it appears that the growth in bike commuting may have plateaued over the past couple of years. Cities with a initially low bike commute share, such as New York City or Washington DC, continue their growth, but once the rate reaches around five percent, growth appears harder to attain.
I am not going to address the potential explanations for this plateau effect, but feel free to put forward your theories in the comments, especially as they pertain to the specific situation in Wisconsin and Madison. In the meantime I will try to get access to the Madison bike counter data, which would allow some cross-validation of the ACS data.