I'm not familiar with mapmaking/coding, but from observing vanilla maps it seems like traffic spawns equally on all road types. This works fine for things like freeways, main roads, and city grids, but it's a bit awkward to have 10+ cars on backroads, access roads, and trails, especially if they are driving cars unsuitable on that type of terrain. Essentially, if there would be a way to set priority/limits on traffic generation on specific roads? i.e. increase traffic density on busy roads, reduce traffic on infrequented roads.
agree this is a very annoying problem especially on west coast where the highway has 5 something lanes you get very sparse density there and jams everywhere else. wish there was a traffic density option for decal roads, with the current option "traffic cars" becoming a "max traffic cars" that limits traffic active for performance reason, but doesn't jam every trail and junction and secondary road.