Back in 1990s I owned a brand new flat almost under the Heathrow flight path, and that had secondary glazing, which was about 15cm inside the double glazed units and was very effective.
The key seems to be you also have acoustic insulation board on the sides, top and bottom between the normal and secondary window frames, as that seems to damp sound vibrations from passing through the frames.
If you really want to cut the noise need at least 3 layers of glass. Also if it is just one side of the house that is noisy there are acoustic panels that can be fixed to inside of outside wall (depending on thickness and type can cut sound by upto about 18db), 10db is halving sound as it is a logarithmic scale. Only downside is will make room marginally smaller.
If noise is big problem get new triple glazed windows and keep the secondary glazing. Will obviously reduce heating bills too, even more so if also line inside of exterior walls. (Whatever shady salesman say, don't waste your money getting new double glazing, if doing the job, specify triple, or don't do the job). Incidentally I believe mulling changing building regulations to triple glazing for new extensions and homes, and as legal changes are always slow, go for the potential future spec.
I have a relative who changed year or two ago, from about 30 year old double glazing to new triple glazed windows, they say house is warmer and heating bills more than halved. No drafts, and quieter too. Admittedly they did whole house but saving around £1500 per year on heating which shows benefit of good triple glazing
