--drive-on-left, --drive-on-right -- Could this be included into the country abbr list we have in the resources - there are not so many drive-on-left countries anyhow, so we just add them, and assume drive-on-right for all other countries. Option is a very bad idea, because you can't use it for a map of europe (or other multi country regions).
You cannot use it also if data of two countries - one with drive on left and one with drive on right - is in the same tile. When both options are not set and --check-roundabouts is set mkgmap makes an autodetection by analysing the direction of the first roundabout. But this detection stores the information in a static variable so when using max-jobs > 1 the drive-on-xxx info might be interchanged between tiles. WanMil