If you live on a busy road, the useful question is not "how loud is the street" but "how much of the street reaches the bed." 

And the answer depends on the whole envelope, not one element of it:

• Glazing is usually the weakest link - but only until it isn't. An ordinary window in a solid wall is almost always the first thing worth fixing. What tends to get missed is that once the glass is upgraded, the weak point simply moves. Sound behaves like water: it finds the lowest remaining path, and there is no benefit in sealing one at a time while ignoring the rest.
   
• The gaps around the window are their own problem. Perimeter sealing, and especially roller shutter boxes above the opening, are a classic flanking route - a thin, hollow casing set into the wall directly over the glazing. Many facade retrofits underperform for exactly this reason, and the fix is cheap compared with the windows themselves.
   
• Ventilation has to be solved at the same time. A well-specified window that has to be opened at night for fresh air is, acoustically, not there at all. Acoustic trickle vents or mechanical ventilation are not an accessory to the upgrade - they are what makes the upgrade real once people are actually sleeping behind it.
   
• Facade walls matter more than people expect. Older masonry with thin plaster, lightweight partition construction, or a poorly built external insulation layer can transmit noticeably more than the wall's thickness suggests. An internal lining system, like an independent or resiliently mounted layer with an absorbent cavity, raises the wall's performance without touching the facade from outside, which also keeps it viable in buildings where the exterior cannot be altered.
   
• The top floors have a fourth side. Traffic noise arrives at a shallow angle from above as well, and a converted attic or a lightweight roof build-up is often the quietest-looking, worst-performing surface in the flat.
   
• Low frequencies travel through the structure, not just the air. Heavy vehicles, trams and buses on an uneven surface produce a rumble that airborne insulation alone will not fully address, because part of it enters through the ground and the building frame. Where this is the dominant complaint, the answer moves toward structural decoupling and vibration isolation rather than added mass at the facade.

Room use is the free intervention. Where a choice exists, putting the child's bedroom on the quiet side of the building outperforms most retrofits and costs nothing but a weekend of moving furniture.

And it is worth being clear about what absorption can and cannot do: acoustic panels improve how a room sounds from the inside, but they do not stop the road from getting in. Traffic noise is an insulation problem, not an absorption one. Confusing the two is the most common and most expensive mistake we see.