I suppose the answer is going to depend not only on the humidity but on distance, terrain, vegetation/surroundings and on the body in question (both physique and acclimatization). For me, somewhere between 115 and 118F and moderate humidity begins to be uncomfortable but I have seen people who really suffer at 90F. I think it is much more difficult for people who have always lived in places where the temperature stays below 80F for most of the year because they never really acclimatize.
It sure helps if the roadways aren't crowded with concrete buildings and parking lots but have green buffers. Unfortunately, that is contrary to the new densification Kool-Aid. I fear we are turning our urban heat islands into urban ovens. Or, considering the toxic reactants the cars add, urban ozone cookers.