The N125 is the road to use to avoid the tolls which are resented in Portugal and the motorways are eerily deserted since the introduction of tolls. The N125 is signposted as such, just watch out for the turn-off as you drive from the airport, it is a bit too easy to miss it. All white signs for Almancil, Albufeira, Lagoa, Portimao, Lagos and even Silves or Sagres are going your way. A point to bear in mind is that if you see a blue sign for any of these along the way then it is OK to follow that so long as there is also an indication on a white sign either for the N125 or else one of the above towns - the motorway will then separate later. As already said, it is slower and it is mostly a single carriageway but it is the preferred route by most people. Only the hire company can answer the roof rack question. You can phone them cheaply by going first to moneysavingexpert.com and using their callchecker for a cheap number to use for calls to Portugal. Enjoy your holiday.