Gosodwyd terfyn cyflymder o 30mya ar gyfer ardaloedd preswyl cyn yr Ail Ryfel Byd, pan oedd llawer llai o geir ar y ffyrdd, a gosodwyd terfynau cyflymder heb yr holl gyfoeth o waith ymchwil a data sydd gennym nawr. Mae gwaith ymchwil yn dangos bod y mwyafrif o yrwyr yn cadw at derfynau cyflymder ar strydoedd preswyl. Dangosodd canlyniadau o waith monitro cyflymder un o'r cynlluniau peilot 20mya yng Nghymru mai dim ond tua 6% o yrwyr yr oedd angen cymryd camau gorfodi yn eu herbyn ar ffurf cyngor, cwrs addysg cyflymder neu gamau eraill.5. Nid yw'n derfyn cyflymder cyffredinol
30mph speed limits for residential areas were set before World War II, when there were far fewer cars on the roads and speed limits were set without the wealth of research and data that we have now. Research indicates that the majority of drivers observe speed limits on residential streets, with findings from speed monitoring work in one of the pilot 20mph schemes in Wales showing that only around 6% of drivers required enforcement action to be taken against them, in the form of advice, a speed education course or other action.5. It is not a blanket speed limit