[OSM-talk] Statistics on road network length?

[Jo]

> Concerning the issue with dual carriageways, I am using a brute-force
> approach and divide the calculated distance by 2 if these conditions are
> true:
>
> highway ~ /^(motorway|trunk|primary|secondary|tertiary)/ &&
>  oneway ~ /yes|true|1/
>

You probably want to add oneway=-1 to that list. There are some
oddballs who used that indicate oneway is against the vector sense.
Also JOSM proposes to use it when reversing a way.

Jo