In Denver (in my neighborhood at least), the difference in assessed lot value between a "Superior" (park front, or the like) and "Inferior" (fronting a one way street) is about 25%. The lot value could be half the value of the home, or of course far more or far less (depending on big and fancy house on small lot vs small and dilapidated house on big lot). So 1/8 of the value of the property might be a reasonable starting point on average but the real number will vary dramatically depending on the particular property.
Take this all with a grain of salt. The city assessor says that the houses fronting Wash Park on Downing St. are "Superior". Park views are super nice, but that's a pretty busy street. So obviously this rating process is subjective.
The county assessors' mathematical models ARE based on actual sales prices (by real sellers who found real buyers) for similar properties. Every property is unique (and so valuation is someone subjective), but looking at similar sales (of busy vs non busy streets) should be at least a starting point.
All this information (for Denver at least) is online. http://www.denvergov.org/Property/
Some commercial assessors may have better mathematical models than the city, but everyone (including you) has access to the same property and sales data.
I'm not a realtor. I just live in Denver with a park view and spent a lot of time investigating sales price information so that I could understand my property taxes.
There is a cost to sell a home with location issues. I would try to find similar homes that recently sold with the same or similar location issues and compare them to sales on quiet streets and cul-de-sacs. To someone coming from a busier street, it's not a big deal but to others they may like the home online but turn it down when they see the location.
One factor will also be what the competition in the price range looks like, on busy streets or not. If you have no inventory, you may do better than you would expect to do otherwise.
Expect a longer time on the market and a somewhat reduced price due to the location.
I agree with the notion that buyers tend to overlook flaws when shopping in a seller's market. They can cost you. In a rising market, that's not a good course to take.
You may want to lower your sights. That is include more homes in your search.
Check with the planning office. Sometimes a noisy street might have a future that will change. Third Avenue in Longmont was one such street. While some complain about the heavy amount of traffic on the west side. The east side improved dramatically when Highway 119 took the majority of traffic off of Third Avenue.
There are some great answers here, and most important to a buyer or seller now might be the fact that in a seller's market there might be no discount for a listing on a busy street. In a buyer's market, where there might be 5 times as many listings for buyers to choose from, a busy street could make a home unsalable.
What is it that you are trying to get out of this house, and what area are you looking in? The specifics, in this case, are very important.
There is not a set deduction for a home being on a busy street. It will depend on the area and how busy the street is. It will also depend on how much a buyer thinks that so called discount should be. If the house is in great condition, has good curb appeal, and you can't hear substantial noise when inside the home, the buyer may put less weight on the home being on a busy street.
Berkshire Hathaway HomeServices
Real Estate of the Rockies
In this market the percentage rate is different and most likely less than before. I would say that depending on how busy the street is and how strong the community it could be as low as 5 to 10%. This is a hard call to give an exact amount. This would be a good question for an appraiser. After all, they have the final say in these matters.
Robert McGuire - 303-669-1246
Best of luck to you.