Cymru fydd y rhan gyntaf o'r DU i gyflwyno terfyn cyflymder 20mya ledled y wlad wrth i'r rhan fwyaf o ffyrdd preswyl sydd â therfyn cyflymder o 30mya newid i 20mya ddydd Sul, 17 Medi.
Wales will become the first part of the UK to introduce a nationwide 20mph speed limit as most residential roads, which currently have a 30mph speed limit, switch to 20mph on Sunday, September 17.