You could build your own DNS set up for Traffic Director. Based upon the recursive server IP hitting you, send back responses that are closest to the user.
Which Google DNS uses to tell your name servers more information about where the client is located. This allows you then direct them to the nearest server.
There is an RFC to pass information about the client subnet: https://tools.ietf.org/html/draft-ietf-dnsop-edns-client-sub...
Which Google DNS uses to tell your name servers more information about where the client is located. This allows you then direct them to the nearest server.