Short answer: yes, engineered acoustic planter boxes exist, but ordinary decorative planter boxes are not soundproof enough for a busy street. No planter alone is truly soundproof; the realistic goal is traffic-noise reduction. A planter box works as a low barrier: its mass blocks some sound, soil and dense foliage absorb and scatter it, and a continuous line of planters can break the direct path between tires, engines, and nearby homes or sidewalks.
For a busy road, look for modular acoustic planter systems rather than lightweight pots. Effective options include high-mass concrete or steel planters with sealed joints; deep soil volumes; dense evergreen hedges; integrated absorptive panels or mineral-wool cores; and planter-barrier hybrids designed by an acoustic engineer. Height and continuity matter more than plant choice alone. A run that is 1.2–2.0 m above the road surface, continuous with no gaps, and placed close to the noise source or the receiving facade will perform best.
In many urban projects, well-designed planter barriers may cut traffic noise by roughly 3–10 dB(A); they work better when combined with a solid barrier, speed reduction, and upgraded windows. They are unlikely to make a very loud arterial street quiet on their own. Also plan for maintenance, drainage, root space, structural loading, pedestrian clearance, and sightlines at intersections. So if your street is truly busy, choose an engineered acoustic planter barrier and treat it as part of a broader noise strategy—not as a magic soundproof box.