Quote Originally Posted by Runyan99
I would suppose that most shopkeepers and merchants and such took it upon themselves to hire private security which would protect themselves and their assets.
From what I know, the late Empire saw individual land-owners (a semi-feudal form of land-owning, the colonate) with private armies.

Prior to that, private security was probably very common. An additional reason for danger in streets was the complete lack of street lights. According to Rise and fall of ancient Rome, the only city in the Empire with street-lights was Antioch.

That probably meant that very few people took a midnight stroll in the city.