ptatohed
Licenced to Spell
MUTCD 2009, page 688 & 689, figure 6H-28. Sidewalk detour. The minimum is 36 inches
What's funny is I don't work with the MUTCD, I work with the CA-MUTCD, right? Well, it is 48" in the CA-MUTCD. I forget sometimes that California ADA guidelines are more restrictive than Federal. Obviously the exam would test you on Federal, but that is why I was thinking 48".
http://www.dot.ca.gov/hq/traffops/engineering/mutcd/pdf/camutcd2014/CAMUTCD2014.pdf (pg 1201)