My first job out of college was in an office that had been converted from an old train depot. Trains went by, within inches of my office window, several times a day. The first time it happened, when I was there for my interview, I was caught off guard in a spectacularly embarassing manner. But I got the job, and after a few more trains, I pretty much stopped noticing them. My point is that even noises that seem jarring at first probably won't register after you become accustomed to them.
A lot of buyers hate the idea of street noise in their home. I understand that it's not anyone's favorite sound, but unless you're buying a secluded farmhouse, there are going to be noises. And you'll get used to them, and they won't bother you. They'll be white noise. My house isn't on a major road, but there are kids playing outside all the time. There are shrieks and screams and typical kid noises. And there's a big, barking dog at the house next door. Inside the walls of my home, these noises don't really disturb me. If I don't want to hear these noises at all, a quiet radio is enough to cover it.
All houses will have some noise. If it's not cars on the street, it will be a train in the distance, a neighbor's dog, or kid, or motorcycle, or cows bellowing somewhere nearby. You simply can't eliminate the noises from the outside world — but you do get used to them, and it doesn't take long to adjust. You hear what's important and you tune out the rest. I don't think street noise should be a dealbreaker for buyers. You, as a buyer, may disagree with me, and that's okay — my job is to make YOUR priorities MY priorities. It's not going to be my house, after all. I'd just hate to see you pass up a great home for something that really isn't as big a deal as you fear. After all, millions of New Yorkers sleep soundly every night despite sirens and horns constantly howling below their windows.
How much does street noise affect the comfort and value of a home? In most cases, not much at all.