Why not just have a maximum speed limit of 20 km/hr?

Stuff reports:

Reducing speed limits on open roads and urban streets could do more harm than good and actually make them less safe for users, motoring group the AA believes.
Long-term road safety data from NZ Transport Agency’s (NZTA) MegaMaps tool suggests speed limits on almost 90 per cent of the country’s roads are too high and should be drastically reduced.
It also suggests cutting limits to between 60kmh and 80kmh on most open roads would improve safety, and that only 5 per cent should retain the current 100kmh limit, the NZ Herald reported.
Reducing limits in towns and cities to 30kmh to 40kmh would also have safety benefits, NZTA suggested.

Of course reducing speed limits reduces accidents. Just as closing down workplaces would reduce workplace accidents. But the questions should be, what is the trade off.

If the open speed limit was 60 km/hr how many more hours would people spend in traffic? How much more expensive would it be to devliver goods for export?