Locking your bike to signs is unsafe as thieves are able to remove the bolts from their bases with power tools and stealing your bicycle in as little as 10 seconds. Taking the time to locate a bike rack is worth it - and Rackster makes it easy by displaying the locations of every bike rack in Chicago within half a mile of your location. Additionally, unlike apps such as Bike Rack Locator, Rackster uses official city data instead of crowdsourcing for the best possible accuracy and completeness.
Currently works only in Chicago - more locations are in the works!