Hmm. There was an article online a few months back reguarding a study of people in the workplace.
The study found that the constant noise pollution of a typical office (fax machines, phones ringing, constant typing, clicking, computer beeps, etc) lowers the average IQ by something like 10 points.
An example for the purpose of demonstrating in the article was that Marjuana was found to reduce the same worker's IQ by only 4 points.
I'll see if I can find the article again, not very likely though. n.n;
Both are temporary IQ loss according to the article, but extrapolating from that: Children in general are deeply effected by everything they exerience in the first few years of life, especially during the first 2 years. I imagine that traffic noises would be significantly detrimental, though I suppose that depends on just how much of an effect one considers detrimental.