Like a few have have mentioned, California in some cases is more (or should I say "many") restrictive with ADA regulations than the federal standard. If your project is in California, I would suggest using the CALDAG....its a great resource. It contains interpretations and explanations of ADA standards between Federal and California standards completely filled with tables, figures and detailed summary of the most restrictive standard.
I work primarly in the San Francisco Bay Area, and yes the great manjority of the time, the sidewalk will follow the slope of the street. That is what is done in San Francisco. In other cities, they allow you to follow the grade of the road as long as the sidewalk is not separated from the curb.
In my 5 years of experience, I have only come across one project where I was asked to meet ADA standards for a sidewalk on a street that exceeded 10%. The only way to do so, was to separate the sidewalk and slope it to the maximum 8.33% with handrails. It was a so NOT attractive solution but it was approved.
The moment that your walkway exceeds 5% and its separated than it becomes a ramp and there are length, width and handrail requirements that need to be followed. I hope this helps.