Mae'r dystiolaeth yn dangos bod allyriadau nitrogen deuocsid ar eu hisaf pan fydd cerbydau ysgafn, sef y rhai sy'n cyfrannu fwyaf at yr allyriadau, yn teithio ar gyflymder o rhwng 40 a 50 milltir yr awr. Mae disgwyl i'r terfynau cyflymder hefyd esmwytho llif y traffig wrth i gerbydau deithio ar gyflymder mwy cyson.
Evidence shows that nitrogen dioxide emissions are at their lowest when light vehicles, the main contributors to emissions, travel at speeds between 40 and 50 mph. The speed limits are also expected to have a smoothing effect, with vehicles travelling at more constant speeds.