[AccessD] Determine Mileage Between 2 Zip Codes

Does anyone know of any code or software that I can use to try and get
mileage between 2 zip codes?  The situation is this, in an insurance
database there is a contractual obligation for my client to report on the
number of members of the plan who live within 7 miles of one of the
providers of the plan.  There are about 250 providers in the Washington DC
area and approximately 15,000 members so the more automation I can get in
this the better.

Thanks,
Chris Mackin
www.denverdb.com
Denver Database Consulting, LLC