The Highways Agency proposed a 60mph speed limit on a 32-mile stretch of the M1 as part of an attempt to ease congestion.
The proposed reduction is part of the "smart motorway" scheme, which would involve the hard shoulder being used as an extra lane.
But the added volume of traffic would risk violating EU restrictions on air pollution.
Lowering limits would reduce amounts of pollution emitted by vehicles at high speed and reduce polluting traffic jams, the Highways Agency believes.
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