Who roamed ancient Roman roadsides waiting for unwary travelers during the medievel period?

4 answers

robbers, highwaymen, vagabonds
http://en.wikipedia.org/wiki/Highwayman

Sometimes they were ex-knights whose lords had been put out of commission in some way. This type of thing occurred in medieval Japanese society, too, when samurai were suddenly "at loose ends" if their lord were killed or captured or whatever. These robbers were called ronin.

Would some of these people be runaway slaves or serfs?
Yes, they could be runaway serfs or slaves.