Within Atlantis, landowners are responsible for maintaining the roads on their land. On the whole, they do not do a particularly good job of this, which is why most people travel by water instead.
For local colour, you can have NPC visitors to Atlantis grumble from time to time about the poor quality of the city's roads.
In the cultivated areas around Atlantis, landowners are responsible for maintaining the roads on their land and keeping them free of monsters. On the whole, they do a reasonable job of this, particularly in the areas which cannot be reached by water.
Many landowners, particularly those who live in Atlantis rather than near the land they own, will offer to hire adventurers to watch over their lands and keep them free of monsters. If the player characters are approached, they would be wise to decline such offers. The pay is reasonable, but the work is boring. Monsters have more sense than to come too close to Atlantis.
As far as player characters are concerned, this is the interesting one.
This map shows how the residents of Atlantis think the road network which connects Atlantis with other cities on the continent ought to look. The light purple lines are roads capable of carrying heavy traffic. The dark purple lines are rivers capable of carrying heavy traffic. It is important to a lot of people with money and/or power (the wizards, the merchants, the aristos) that the roads remain open, well maintained and free of monsters.
Well, that's the theory. The reality is that, particularly in the mountains, the roads are neither as straight as they look on the map nor as well maintained as they ought to be. The problem is that no-one is responsible for maintaining the road network.
The way this problem is resolved is that road maintenance consortia provide quotes for maintaining particular sections of road for a year. Anyone can provide a quote, and the lowest quote gets the contract.
Some sections of road are easier to keep free of monsters than others. This side of the mountains, unintelligent monsters will occasionally stumble onto the road and make nuisances of themselves, but intelligent monsters know that if they cause any trouble they will have the wizards of Atlantis after them. In the mountains, and on the other side of the mountain range, anything goes.
And this is where the player characters come in.
Introduce a new player character at this point, most likely a fighter, starting off at 2nd level. Before becoming an adventurer, he worked for one of the road maintenance consortia, but he has recently left it, perhaps because of a personality conflict.
Possibly the existing consortium failed to get its quote in in time, or possibly it quoted too high a price. What has happened is that the new player character has, unexpectedly, been given the contract to repair a section of road and keep it free of monsters.
Keeping the road repaired is not a problem. Through working in the trade himself, he knows what needs to be done, and through his contacts in the trade he can also find enough semi-skilled workmen to do the job for wages which will leave him with a nice profit.
The problem is that the section of road is not free of monsters. His section of the road is this side of the mountain range, so intelligent monsters do not trouble it, but it is plagued by more than its fair share of unintelligent monsters, ranging from oozes and slimes to packs of wild dogs. I may provide a wandering monster table in due course.
Anyone would think that his rivals in the road maintenance business are arranging it deliberately. They wouldn't do a thing like that, would they?
If our new player character does not meet his contractual obligations, he will be thrown in jail and/or sold into slavery. In Atlantis, either of those is very bad news. But since the price he quoted for maintaining the section of road was a low one, he cannot afford to hire serious professional adventurers.
So what he plans to do is raise a party of novice adventurers, leading them himself. After all, how difficult can it be to clear a section of road of unintelligent monsters?
Raising the party should present no problems at all.
Fighters? He can always find first-level fighters in the building trade. Additionaly, he might be able to recruit someone from the aristocracy. The second option would be better, as it wouldn't cost anything. Young male aristos would not work for pay - it would be beneath their dignity - and there are always young male aristos looking for adventure. (This would be a good way to introduce a second, contrasting, fighter player character.)
Clerics? He attends a temple regularly, and if he makes a modest donation to temple funds and explains his problem the high priest should take the hint and instruct a first-level acolyte to accompany him on his adventures. (Which temple it is that the player character goes to regularly is up to the player.)
Rogues? There are one or two young ladies who until recently have been working at the Comedy Theatre and who are known to be looking for adventure. They look as if they have their wits about them. (If the player character dancing-girl and Nylchrinie left Xukry's show in Atlantis, they are the two in question. If they are not, there are plenty of other dancing-girls from Xukry's show.)
Magicians? Easiest of all. There are always student magicians at the Institutes looking for some way of earning their course fees. (This would be a good way to introduce a specialist wizard player character.)
Note that this is a good start to a long-term campaign. If the party is successful in keeping their section of road free of monsters (as they should be) they can go on to quote for other, more exciting, stretches of road further afield.