The transit authorities in Catalonia, Spain have decided to take the drastic speed limit on the accesses to the city of Barcelona
to the next level. Similar to plans in other cities, like Rotterdam in the Netherlands, highways in Barcelona will now have variable speed limits. A smart system with light panel indicators will regulate speed on the C31 and C32 highways: the higher the pollution levels, the lower the speed limit will be. The system also applies a dynamic approach depending on congestion: speed limits will be reduced to avoid traffic jams at bottlenecks, sometimes all the way down to 40 km/h (25 mph). Some groups have asked for the speed limit to be raised when the conditions improve, but so far, the maximum speed will be kept at the current 80 km/h (50 mph). On a normal highway in Spain, the maximum speed limit is 120 km/h (75 mph). Up to 40 percent of the pollution in Barcelona is a direct consequence of traffic.
[Source: El Periódico