So, [you] set a number that is "safe":
- consideration of visibility distances
- junctions etc
- likely other road users / those nearby (school zones, resident parking)
and so on
[You] also have to factor in what is "reasonable", which is basically what speed a sensible and competent driver* would drive: otherwise, it'll just get ignored anyway, and be self-defeating.
* no point considering the nutters, as they will do what they feel like anyway, regardless of what a number-on-a-stick says. Just catch and jail the foookers.