Auto-redirect to closest/fastest mirror

I am trying to add automatic redirection for a large website that already has three mirrors, and will soon have more. It will become impractical for users to always manually choose the best mirror site for best performance, so I want to set up a wayto send them immediately to the correct mirror site, either based on geographic clues or on ping speed. I’ve searched for a couple of hours into “ping triangulation”, “echo location”, and “diverse routing” but seem to find only vague ideas from several years ago. Does anyone know what works and how to learn more?

David Booker
German Cochrane Centre