An environmental expert says there's an extra benefit to lowering speed limits- a reduction in carbon emissions.

The argument being made is that cars that drive at slower speeds use less fuel and emit less carbon dioxide, which is better for the environment. 

AUT professor of biogeography Len Gillman says SUV and other high-emission vehicle drivers need to do their part to cut CO2 output.

Len Gillman says reducing speed limits is more equitable than taxing fuel, because low-income drivers will be hit the hardest.

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