Lanes generation potential is far more limiting than carriageways

[liberostelios]

There are far more limits to procedural lanes generation than in carriageways. This is because lane have a much more local variability nature than carriageways. For example:

In other words, lanes vary across the same road much more than carriageways. Creating really useful lanes (especially merging lanes etc) requires more detailed width calculation (e.g. a width per segment of the same road) which are also more prone to errors.