"How much does proximity to a busy street affect the value of a home? How about power towers?"
Busy streets do affect a property's value, some more than others. There would be a large adjustment if a home backed up to a main road near a mall where there is a ton of traffic all day and night. Not as much of an adjustment if it backed up to a smaller feeder street that had traffic mostly during the morning and evening rush hours. How much effect also depends on sound abatement, intervening landscaping, distance to the roadbed and other factors. The only accurate way to get an adjustment is to do a paired sales analysis on homes that are near a main street and those that aren't because it differs for every neighborhood. In some areas the adjustment might be small, in others it might be fairly large.
The same holds true for power lines, but mostly just for the high tension lines. And it depends on the exact distance. If a home is within the "fall line" of a tower, the adjustment would be far greater than if it was just within sight. The impact of high tension lines is hard to measure because so much of the buyer's reaction is based on myth and preference and has little to do with measurable ill effects. Once again though, the only way to come up with an true adjustment is to do a paired sales analysis for that particular neighborhood, homes within a certain distance of power lines and homes that are more removed.
I know that probably isn't really the answer that you were looking for, but the truth is that we do not have a rule of thumb to make those adjustments. The adjustment must always be based on market driven analysis if the result is to be credible.
Thanks for asking though. I love the chance to put into words the concepts and methods that we utilize.