Assuming that there will be highway men in this game, they should be A LOT harder than they were in Oblivion. My first time meeting one of them I thought to myself, "Oh Crap! I better give him my money." But I soon began to miss my money so I decided to try and kill him. And much to my dismay, he was just as hard as a regular bandit. If highway men are in Skyrim, Please Beth, actually make them something I fear.
I think they should actually flee. Just like in reality, a thug wants money and not to get in a fight. If you were a thug, wouldn't you rather mug an old lusty argonian maid rather than the champion of Cyrodiil?
Heres a story: My buddy got surounded in an alley by three thugs. He had a hammer which he was using to fix the electrical unit. They surrounded him, so he began brandishing the hammer at them. To his luck, the rubber hammer-head flew off and stuck one of them in the neck. Here he is with a wooden stick, and they all panick and run. (leaving the thug with a hammer head in his neck to bleed to death).
So, these highwaymen should have high aggression, but a very low willingness to fight.
EDIT: Yes, i agree with the poster below. Not all should run, but some should, depending on your fame/infamy. Even then, some fearless brutes should still challenge you.