Portuguese road toll, "electronic only"

There are some highways in Portugal on which tolls are "electronic only". It means that there are no cabins and the passing of vehicles is detected by devices placed at the beginning of those highways; you will find road signs announcing with "Electronic toll only" (similar to signals above).
It is possible to pay Portuguese electronic tolls online, before you leave home, on the following website: 

Most of these highways were free of charge; however, due to the crisis affecting Portugal, among other countries, the Portuguese government had to introduce the electronic tolls. Apparently it was the only solution to keep within the budget and ensure road maintenance.  

Electronic tolls in Portugal