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If not hearing the traffic (or birdsong) is you priority a few thoughts about double glazing might be useful. Ordinary double glazing units do cut out a lot of sound but they are really designed to stop heat loss. With an air gap of around 16mm, great for thermal insulation, there are some sound frequencies that are not cut out very effectively. Unfortunately, such windows are better at cutting down the high-pitched twittering of the birdies than the lower rumble of lorries. And the wind will seem to moan rather than whistle.
The wider the air gap, the more effective will the sound proofing be, so secondary glazing is more effective than sealed units. The thicker the glass, the more effective will the sound proofing be, but a better result is achieved if laminated glass is used. 6.4mm laminated will stop as much sound than ordinary glass at twice the thickness. You can get laminated glass with especially thick plastic layer made specifically for sound insulation. Its called 'acoustic glass'.
Don't underestimated the ability of sound to get round corners and squeeze through small gaps. Really tight draught-proofing should always be the first approach to improving the sound-proofing of your windows.
So for practical, cost effective, soundproofing, first make sure the draught seals are tightly in place, then consider secondary glazing with laminated glass leaving as wide an air space as you have room for.
There is lots of really good technical information and advice available from Custom Audio Designs.
Lincolnshire
©Biff Vernon 2005