Because a lot of people are migrating in a collapse from urban to suburban to rural areas, there will be highwaymen too. Brazen ones will set up among the congestion and exact tolls. Since one stalled party ahead slows everyone behind, some security will initially happen, but as things decline, then those soldiers depart and even if a lot of people are armed, then it might be easier to pay the “toll”.
It's way easier to collect a toll than deal with a mass of irate travelers. Hard to replace the highwaymen's crew unless there's a lot of ex-cons, hungry soldiers, or outright criminals around. Oh wait, there's lots of them in urban areas, isn't there?
Worse ones will find stragglers who got late starts and are isolated and weak and seize goods or worse. If you look at maps, there are obviously places they might be. Not usually a stone's throw from the town, but a bit further ahead. Often they'll be living close by, somewhere that isn't immediately obvious, but they need meadows for their horses and fresh water and firewood.
If they create only a nuisance, since they know the terrain very well, they'll be mostly ignored. But if a constant threat, then some brave souls will repel them, or the locals will, or they'll be killed by their own outlaw band members who get a hankering for leadership. 'Might take awhile.
This means someone's riding “shotgun”, not just calling it out like people do now, but actually prepared to defend and be alert for ambush and pinch points. There's always natural areas, but even a covered exit may create a temporary place for bandits.
They'll be lots of wannabee highwaymen. The survivors will get better at it. Long term, they'll become experts at it.