Every large metropolis has it's 'rising crime rates' and Nairobi is no exception.
So when you go at night into the emptier or deserted streets of the city, go in group or at least a pair. Strangely enough area notorious for being most dangerous is probably safest - River Road area. This part of town constantly buzzes with activity. Crowds on street until 3 am over weekends. Most films are US, English or Indian romance-type.