Infrastructure Minister Michelle Haywood says research into 20mph limits isn't conclusive on whether it'll affect greenhouse gas emissions.
She faced a question for written reply from Arbory, Castletown and Malew MHK Jason Moorhouse about the proposed new limits' effects on buses.
In her reply, Dr Haywood said research showed there was unlikely to be any major delays to bus services, and there were no studies about passenger numbers.
But she added one model found lowering limits from 30mph to 20mph would significantly reduce some emissions - although others had found little change.
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