Best new hedging to place within existing Hawthorn hedging

[lindsayogle]

Hi I have a property near a main road. The hedging is existing old Hawthorn, up to say 1.5 metre high but has many gaps. I know I cannot rid myself of the traffic noise but I could reduce it and at least block out the images of cars driving past if I could thicken the hedging. Length affected- circa 25 metres. So I was thinking of adding a fast growing/dense species to intermingle within or in front of the existing hawthorn-( the latter-so the Hawthorn would be seen from the road, but the new hedging from the garden ) eg. Thuji Emerald, Castelewellan Gold, Laural hedging or Leylandi Green. Have I missed any others? What would be best? Need fast growing. Need dense. Ideally something that could grow within the gaps, rather than in front of existing.
Thanks.