Great question! Wish I had a great answer. There is no one formula that we can apply to figure this out. If you are working with a Realtor you should ask him or her to run comps of every property that has sold on the side of this particular road foar as far back as the MLS will permit. Once you gather that data look for comparable house in the same jurisdiction that sold in the same time frame that were not by the side of the subject road. From that data you may be able to derive rough idea of the percentage of decrease in value for the road factor. Of course that assumes the road was relatively the same in the subject period as it is now, which it probably wasn't so be sure to err on the side of caution when you apply your % deduction for the busy street .
Best of Luck to you
Carole & Greg