According to the institute, of the 24.2 billion places per kilometre provided, only 23.2 percent were actually taken up with this ratio down 0.9 percent on 2013.
This performance nevertheless leaps substantially when taking into consideration international road passenger transport that returned an occupancy rate of 74.8 percent.
However, there were more Portuguese citizens taking to the skies with air transport rising by 9.4 percent, up from a 4.9 percent increase in 2013, with a total of 35.7 million passengers.
Of this total, Portuguese registered carriers provided a total of 16.5 million places on board, up 3.5 percent on 2013 while actually carrying 12.7 million passengers, up 6.7 percent on 2013.