Evidence seems to show that usually the only significant improvement from a hedge is psychological - which is not to say it is a bad idea.
Secondary glazing can work well so long as the windows are shut (!), the walls are fairly heavy, and there are no other significant leaks. Make sure the outer windows are really well sealed (you may want a very small deliberate leak to reduce condensation) and make the air gap between the two panes as large as possible. If the air gap cannot be large you could use heavier glass (6 - 10 mm). Avoid thermal double glazed units which are worse at low frequencies than solid glass of the same weight.
Whether the wall will work depends on the geometry of the site. If it gives visual shielding it will work to some extent at least, but watch out for sound coming round the ends. You may not need masonry, you will get some effect from a heavy timber wall with no gaps. Since some sound will get round the wall, the higher performance of the masonry may not be significant. The extra sound reflected to the opposite side of the road may be detectable but will probably not be very significant unless your wall is quite close to the traffic. You may have to be careful if there is a barrier on the other side, you should make sure your wall does not reflect sound over the barrier to windows that would otherwise be shielded. You can work it out with angle of incidence = angle of reflection.
Tony Woolf