Martin_Dold
The entries from Hviera83 and Antmadeira do not fit together.
According to Hviera83, cycling is allowed, according to Antmadeira, it is forbidden. Only one can be true. 

in Mapillary, there is no sign visible, that this stretch is forbidden for cyclists, (https://www.mapillary.com/app/user/asimonelli?lat=40.15162711118853&lng=-8.841474923420833&z=17.588090556425136&pKey=VIW0m3IwuVjBCg12m9cnYw shows a sign "forbidden for pedestrians, cyclists, animals etc -", which would not make any sense here, if cycling would be forbidden. 
http://www.estradas.pt shows this stretch in a different colour then the motorway
So I would rather support Hvieira83's opinion.

please verify ON GROUND, have a look to the existing (or non-existing) signs. 