I can't figure out a way to get that average drive time easily.

A much easier solution is to do drive time rings around the two new locations, count members, and express the results as

Location A has 50% of members within 7 minutes
66% within 10 minutes
82% within 15 minutes
91% within 20 minutes

Location A has 60% of members within 7 minutes
70% within 10 minutes
85% within 15 minutes
93% within 20 minutes

And the better location should be obvious.

Gary Denton
#1 on google for liberal news or easter lemming