How do you make an AI only walk on a certain mesh? (If possible)

I’m currently creating a city in Unreal and I want to add some basic ai characters walking along the roads. I’m very new to Unreal so I know very little about ai navigation and the like but I figured that maybe there’s a way to make the ai only walk on certain meshes. In my city (which is about 2,5km X 2,5km big) the roads are divided into only a few large meshes (which you can see highlighted in yellow in the picture) and I wondered if I can use it to my advantage?

Is there a way to tell the ai to only walk along the road meshes without manually placing nav meshes everywhere?

If this question has already been answered a million times then I apoligize in advance. I’m not very knowledgeable with unreal and I’m not even sure what I should search for in the forums…