We arrive at CDG at 8:20am on a Friday morning (if all goes well), and we need to catch a 10:50am train at Montparnasse. The Air France bus would be the easiest and most economical way to go, and a cab is possible, but will it still be rush hour by the time we exit CDG? Another possibility is the RER B to Denfert-Rochereau, and then a cab to Montparnasse. I'd hate to get stuck in traffic and miss our train - so help, please, what should we do? Thanks for the input