RACC has published big advertisements on the local and regional press calling the measure "absurd" and they offer an alternative (variable speed limits according to traffic and weather conditions), justifying this suggestion with their own data. RACC considers the reports that the Catalan Ministry of Environment used to justify the speed limit were wrong.
According to RACC, its German equivalent ADAC made a report a few years ago stating that reducing speed actually reduced pollution levels by 24 percent but increased NOx pollutants by 26 percent, because cars would have to use lower gears with higher rpm. Moreover, this report was made with a balance of 50 percent gasoline and 50 percent diesel cars and the Ministry didn't take in consideration is that the actual percentage of diesel vehicles on the road is far more than 50 percent, increasing the effects of NOx pollution.
RACC says the Ministry's calculations didn't take into consideration that 40 percent of cars usually don't reach high speeds, because they're in traffic jams and constant starts/stops in congested traffic actually raises emissions by 60 percent.
[Source: RACC (link points to a .doc file in Spanish]