It will leave the current A465 between the Hirwaun and Trewaun junctions for about a mile (1.4km) and between the Taf Fechan Viaduct and the Gurnos Farm underpass for about half a mile (0.8km).
Bydd yn gadael ffordd bresennol yr A465 rhwng cyffyrdd Hirwaun a Threwaun am tua milltir (1.4km) a rhwng Traphont Taf Fechan a llwybr i Fferm Gurnos am tua hanner milltir (0.8km).