For routing purposes, and from reading the code (mkgmap) for making garmin routable maps for satnav (garmin being by far the most popular).
And also from personal experience with satnav.
Definations from mkgmap are:
Speed Value kph
Motorway - 7 no limit
Trunk 5 90
Primary 4 80
Secondary 3 60
Residential 2 40
Living 1 20
In Uganda the speed limit is really 80kph even on the motor way !
I would think ideally primary roads will link towns (eg Kampala - Jinja)
Unless a road is clearly a residential or living street.
Seconday for major tarmac roads within the town Tertiary for other tarmac roads within the town or major murram roads Minor for murrum roads
This is the blue print I am using with good results.