20kph - carparks, laneways, shared spaces, school zones, etc - self explantory, except school zones to be marked as is - with white zig-zags.
40kph - suburban side streets/CBD - distinguished by white lines
60kph - Main roads (2x2 or more) - distinguished by yellow lines
80kph - hilly roads/arterial roads/expressways (yellow lines & 80 speed signs)
100kph - country roads/freeways (white lines and 100 speed signs)
Here's my version of what you've said:
30kph - carparks, laneways, shared spaces, school zones, etc - self explantory, except school zones to be marked as is - with white zig-zags.
50kph - suburban side streets/CBD - distinguished by white lines
60kph - Main roads (2x2 or more) - distinguished by yellow lines and 60 speed signs
80kph - hilly roads/arterial roads/major city roads with slip lanes - yellow lines & 80 speed signs
100kph - country roads - white lines and 100 speed signs or open road and unmarked, unsignposted, default country speed limit
up to 130kph - freeways, expressways and other controlled access roads - speed signs to be electronically adjustable and speeds adjusted down as appropriate for environmental and traffic conditions.
up to 130kph - all national highways - minimum road width with sealed shoulders to cater for 3 large vehicles side-by-side*** - lines marked white with cats-eyes, reflective roadside markers, rippled lane edge, signposted as 110, but 130 permitted when safe to do so. Make it law for car headlights to be on 24/7. Major intersections with slip lanes.
*** This is something that I'd really like to see happen nationally, it'd increase safety and help to cater for the idiots who underestimate overtaking, I've seen a few close calls - it'd allow three cars to pass each other with none of them having to go off into the dirt shoulder. On some of the roads today, they're wide enough that one can get half off into the dirt, it might be okay, provided they don't lose control of their car. It would also, for example, give trucks and caravans room enough to pull aside a little to aid in overtaking for cars behind them. Some of the national highway is so narrow and has no shoulder that no one would have any hope.
The only thing you'd have to be careful of with the yellow lines is solely relying on those - there'd be plenty of folk who'd find it difficult to see the difference between the two (colour blindness, etc.)