The safest way there
Every candidate route is scored stretch by stretch for the moment you will actually be standing on it — so a walk that starts in daylight and ends in the dark is scored that way.
It scores the worst stretch, not the average
A route that averages 80 with one 200 m gap at 22 is worse than one that averages 68 and never drops below 55. Ranking is weighted accordingly.
It invents alternatives Google will not offer
Where a detour is plausible we find lit main roads, transit stops and venues open at that hour, and force routes through them to see whether the trade is worth it.
It scores each stretch for when you will be on it
On a fifty-minute walk starting before sunset, the last stretches are assessed in darkness — because that is when you will actually be walking them.