Hopefully, you will miss the tail-end of the Lisbon evening rush hour which can be manic.
I have driven in many cities around the world, and all around Portugal, in horrible traffic conditions, but the A5 Lisbon to Cascais is one of those urban motorways I always avoid as it is a racetrack complete with lunatic drivers!!
If there are evening traffic jams or breakdowns blocking traffic, allow at least 90 minutes from the airport to Cascais.
Also be aware that road signs indicating of off-ramps on hte ring-road suddenly pop-up out the blue. If you are taking the 2º Circular (E01) to connect with A5, make sure, what ever you do, that if you miss the off-ramp for the A5, you do not miss the next off-ramp for Alcantara. If you do, you will end up over the toll bridge on the south side of the Tagus.
