Changes

You can also go to Portes d'Orleans (almost the same than for A6 and A10) and have a sign "Chartres" or "Rennes". If you get a ride to Péage de Saint Arnoult (from someone going to Orleans - A10 - for example) it's good, since it's before the split.

Public transport in Paris consists of metro, RER (fast trains serving the suburbs with relatively few inner city stops), buses and a few trams. If you're paying for tickets and you'll make a few journeys then the cheapest option is to buy a carnet of 10 single tickets (around 11 euros). You can travel to the end of RER lines for around 15 euros.