Right, no “soundproof” curtain will absorb low frequency noise (which street noise unfortunately to a large extend consists of), regardless what ads are trying to sell you. Heavy curtains can absorb high and mid range frequencies quite well and professional acoustic curtains made of heavy (≥500 g/m²) Molton or Calmuc are the best you could get (the cloth costs about 10-15 €/m²), but if honest producers will tell you they won’t help against low frequency noise. Anyway, if your windows aren’t too large you could try that for a few bucks…

The best bet are new insulated vacuum triple glass windows that aren’t cheap at all.