I’ve noticed that many roads that could have been represented by a single 2 way road had been drawn as two seperate 1 way roads.
Which is the better approach? Especially when merging those 2 types together it can get rather messy.
What I’ve noticed so far :
Pros for seperate 1 ways
. Less turn restriction problems, if less than careful, many times it’s forgotten, especially the u-turns.
Pros for 2 ways, single line
. Reduce data usage, can look cleaner
Did you mean, it should be like:
rather than like this?:
From the wiki, divided highways (or what they say double- or multi-carriageway in the UK) should be mapped separately: Tag:highway=trunk
I reckon it applies to every kind of highway, including the service ones.
Perhaps they need to be updated to show that they are in fact double carriageway highways.
Keep in mind, OSM database is used as well in e.g. Project Open Source Routing Machine besides e.g. Garmin. So, I guess, turn restrictions are more important rather than clean looks.
I vote for the first option for the simple reason : KISS. Any road is physically divided becomes two separate oneway roads, regardless how big the divider is ( meaning that a car cannot physically turn right except at allowed points).
I actually came across this tagging using single lane with multi tags (twoway yes, divider yes, left_turn lane 1 etc etc) in Seri Kembangan area. It is way more complicated than it should be, compared to the way that is usually done. It seems to me that the mapper was carrying over mapping technique from another navigation software into OSM.
See http://wiki.openstreetmap.org/wiki/Proposed_features/lanes_General_Extension if you really want to get complicated, but IMO it doesn’t help other mappers much.
If we forget U-turns, that’s just a minor inconvenience to fix. Sometimes we missed them due to the quality of aerial imagery, and sometimes because we are in a rush. I still find those missing U-turns around Kota Damansara and pretty much everywhere big housing areas with divided main roads exists (JB areas have quite a few too).
In Malaysia, many data are still missing, even some major highways are not on the map. So it is not uncommon that roads with separated carriage ways are still mapped with a single “way” instead of two separate ways. If you discover such places (and have time to fix it): fix it.
Regarding Seremban, I believe that the highway classifications to be a little bit a little higher than it should be. While the primary roads classification seems correct (except for part of Jalan Sg Ujong going south seems unplausible), the secondary type is too widely used. A conservative guide:
primary : roads (not highways or trunks) linking city to city, or city to important towns
everything else is a blur to me too 
but malaysia wiki’s specs are pretty good guidelines.
I’ll try to do what I can, prioritising filling up missing roads first, then try to use it when the maps are updated to see what the differences are.