Here are some helpful web sites with resources which may answer questions you may have about detectable warnings or other ADA related issues:
U.S. Access Board – Federal Agency which oversees accessibility components of ADA.
ADAAG Standards – Americans with Disabilities Act Accessibility Guidelines. The Access Board (link above) issues standard minimum guidelines. This links to those Guidelines.
ADA – Americans with Disabilities Act
FHWA – Federal Highway Administration
US Access Board – Detectable Warnings Update
US Access Board – Streets and Sidewalks
National Center on Accessibility – Indiana University site with topics regarding accessibility
Accessible Design for the Blind Links – Additional website links