The American Community Survey is an annual count that replaced the Census Long Form, the results of which were released in 2001. The ACS became official in 2005 and thus 2009 was the first year that they were able to release 5 year data set. The five year report is a sample that is large enough to drill down the commute to work data to the Census Tract level as well as every county in the US.
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The interactive map above of the all the US Counties is color coded by the percentage of bicycle commuters with the lightest number representing less the 0.1 percent and the darkest representing 10% plus. You can also click on a county to find out the number of people who walk, take transit or drive alone.
The County with the highest percentage of bicycle commuters was Gunnison County CO. Where 10.2% of all commuters travel to work by bicycle. Philadelphia registers at 1.5% however year to year data indicates that the latest commute estimate to be 2.2% the 2005 bicycle commuter mode share was at 0.9% which helps explain the 1.5% mid point number.
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Postcard Perfect Crested Butte, Gunnison County, CO
Data Source - http://factfinder.census.gov