Downtown downtown Detroit ( the "Green Zone", surrounded by the freeway moats) a decade or two again wasn't that great; but it wasn't super-duper, dangerous either. Mostly crimes of opportunity you could run into in any major city. Keep your head on a swivel and don't be urban-stupid.
The neighborhoods are where it gets sketchy, if you're not from there. Or even if you are, in some.
Downtown Detroit since about 2013 has become a playground for the 20 something set. Plenty of white women in their 20's walking alone, riding bikes solo, living, hanging out downtown, all times of day and night these days. It surprised me when I first moved back from Chicago. But now it's how it is.
They're even running out of housing for all the demand to live downtown; Gilbert's turning downtown into a company town and opening a new "micro apts" building for the influx of millennials to Downtown Detroit; so he an get back 1/3rd of their paychecks each month:
http://www.detroitnews.com/story/business/2016/11/03/gilbert-micro-apartments-detroit/93242966/