I know that the Roman Empire had land trade along the Nile River, which provided water. But how about west of the Nile Valley? Did the Romans have any means of crossing the rather dangerous Sahara?

A lot of this will depend on whether the dromedary camel was domesticated in the western region back then. The Romans had donkeys, but donkeys cannot go without a drink for a long period as a dromedary camel can.

The dromedary was definitely used by the later Arabs and Berbers in more recent times in the Sahara, but was it around, say south of Libya 2000 years ago?