I think a website should be created that shows all the locations of the message signs on the highways and what they say because some states like Pennsylvania don't show a list of all the message signs online.
The only way this would work is if users could update it. However, this would encourage updating the website/app while driving, which is dangerous.
For the following western states...
* California
* Oregon
* Washington
* Idaho
* Montana
* Colorado
* New Mexico
* Arizona
* Utah
* Nevada
... go to One Stop Shop for Traveler Information at http://oss.weathershare.org/. Make sure "CMS" is checked under Road/Travel Conditions. Note: VMS info not available in Colorado and Montana (traffic cams only).
Caltrans' QuickMap (http://quickmap.dot.ca.gov/) provides similar information but only within California. Make sure "Message Signs" is checked under Options.
Illinois, Indiana, Michigan, Minnesota, and Wisconsin: http://www.travelmidwest.com/lmiga/home.jsp
http://www.travelmidwest.com/lmiga/dms.jsp and follow the drop down menus.
KC Scout also (ran by both KDOT and MoDOT on respective sides of the state line) has a website for all their signs and cameras: http://www.kcscout.net/
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KDOT also runs http://www.KanDrive.org
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Here is Arizona's statewide map of DMS:
http://www.az511.com/traffic/
Louisville, Kentucky: http://www.trimarc.org/site/pages/Index.html