I'm fairly new to the industry and I've been asked about a home that is about 200 feet from an expressway in a suburb of Silicon Valley. I've discussed the highway distance issue (300 feet) before with peers but I've never been asked about this with an expressway/major street.
The sound aspect is up to a potential buyer's personal preference, but once I got the air quality question, I didn't have a good answer. What little research I could find is that the zip code is in a "good" zone with the usual commute peaks and valleys. The sales records of homes within the same proximity to this expressway show comparable numbers to similar-sized homes 1/2 mile - 1 mile away, so it doesn't look like other people have been concerned about it.
Having lived by there at one point in my life, I know that it's like any major suburb street where it's pretty dead except for commute hours, when it's pretty packed.