Short answer: no, a metal backrest won’t truly amplify traffic noise the way a speaker would. It can, however, reflect some high-frequency sound—like tire hiss, brakes, or engine clatter—back toward the bench. That can make the noise feel slightly sharper or more present when you’re sitting right against it, but it does not add much overall volume. Most traffic noise is low-frequency rumble, and a thin metal panel does not reinforce that.
If you are placing a street bench near a busy road, the metal backrest is only a small part of the noise equation. A solid metal backrest is acoustically reflective, so it can bounce sound toward the seated person. A perforated metal backrest, slatted wood backrest, or open design scatters sound and feels noticeably less “ringy.” For a quieter bench, orientation matters more: angle the bench so people are not facing directly into the traffic flow, or place the backrest toward the road so it acts as a small partial barrier. Adding a hedge, planter, or low wall behind the bench can also reduce road noise and make the space feel calmer.
Bottom line: a metal backrest may add a little reflected high-frequency noise, but it won’t make a busy road dramatically louder. If you want the best comfort, choose a perforated or slatted metal backrest, position the bench carefully, and add planting or a barrier behind it.