The so called libres are the old roads that where there before the 1950's and then the highways where started in the United States and a few years latter they statred building in Mexico the toll roads in the same manner as in the States, four lanes with a median, today they have built a lot more, I remember driving to Acapulco frtom Mexico City on a "libre" and today they have a toll road that shaves several hours from the trip.
It is recomended to use the toll roads because there are more Angeles Verdes "Green Angels" that will assist a traveller with a brake down, the toll roads also have an insurance when you receive the receipt that you have paid, I do not know what it covers, maybe somenone can tell us.
navegator
