Transit visa needed?

[visitor_visa]

My family members are planning to visit me in the coming weeks. As we are looking to book their tickets from India to US, most flights have a stop over in Europe. They have a valid B2 visitor visa.

Is a transit visa necessary if you are traveling (and perhaps changing flights) via London/Paris/Franfurt/Zurich/Amsteram? Will a transit visa be required for either the onward to US or return to India leg?

Thanks a lot.

[immihelp]

Look at http://www.immihelp.com/visas/transi...uirements.html