Actually during the energy crisis in the US in the 1970s, this was done on highways to reduce fuel consumption and in theory, it works as cars are most fuel efficient in the 90 km/h range (generally speaking). I would not support this in any other road as they are slow enough as it is.
I speed on the highways, but am mixed on this...I see a very easy way to to reduce emissions but seems to imposing on freedoms...although I guess you are free to do what you want. Its almost like a user pay system- you want to pollute more, you pay for more gas, more tax, speeding fines, etc.
"I believe that banking institutions are more dangerous to our liberties than standing armies. If the American people ever allow private banks to control the issue of their currency, first by inflation, then by deflation, the banks and corporations that will grow up around the banks will deprive the people of all property until their children wake-up homeless on the continent their fathers conquered."
-Thomas Jefferson 1802