This stuff article from April last year gave that statistic, which works out at an impressive 72 plants per metre of distance, but as we observed in the 6 April post, much of it seems to consist of flax and grasses which takes a while to grow.  However, they will still help absorb the carbon emissions from the traffic.

At least where the embankments are visible from streets, one might have hoped for trees that grow to about the 4-metre level to obscure the view of traffic.

One of the things we have always assumed, but were told recently is not the case, is that trees act as a noise softener.  But this person who seemed to know her subject said that they are too porous for that and only add a bit of white noise to disguise the sounds.

If you want to reduce sound you need a wall — and a tall, unbroken one (shades of Donald Trump?).  This info on the US Federal Highway Administration website gives the principles involved.