You're looking at two related but separate issues. The first is: How much does the proximity to the busy street reduce the value of the property? (I don't call it a "discount" because "discount" suggests that you're getting more value. You aren't. You're going to pay less because the property is less valued by potential buyers.
The second issue is that some people just won't buy such houses, regardless of the price. Let's say the comps in quieter neighborhoods are $500,000. You follow the procedure suggested by Mike and look at the variations between house prices in other somewhat nearby areas--busy street or not busy street. And you apply that same percentage reduction to the house you're asking about. Let's just say it's worth 10% less. (Just making that figure up.) So, the house you're interested in might be "worth" $450,000.
Problem is, if 20 people are interested in the house a few blocks away at $500,000, maybe only 15 would even consider your house, even at $450,000. Maybe they've got small children. Or pets. Maybe they're particularly sensitive to exhaust fumes. The point is: Your buyer pool will be smaller, too.
As TerritoryRE points out, those items will vary by town, city, and state. So there isn't a set number. Just recognize, though, that there's not only the lower value you're concerned about. There's also likely to be a smaller buyer pool.
Hope that helps.