A new study from the University of Washington concludes that, in general, "most poor households would not be substantially affected by tolling, either on all highways in the Puget Sound region or specifcally on the 520 bridge across Lake Washington."

However, for very poor commuters (those making less than the federal poverty line of $17,600 a year for a family of three) who commute by car across the bridge, the annual impact of regionwide tolling would be the equivalent of 15 percent of their annual income---a level four times higher than that of a median-income family---and tolling on 520 alone would amount to 5.5 percent of the typical poor family's income. "Those who use routes to be tolled and do not have ready alternatives will have their economic well-being decreased," the study finds.

But those stats, too, come with a caveat: Very few low-income commuters use the 520 bridge. According to the study, just 2.9 percent of poor commuters use the 520 bridge, compared to 4.1 percent of all non-poor commuters.

"The poor in the Puget Sound Region are much less likely than their near-poor or non-poor counterparts to use a personal vehicle to get to work" and more likely to use non-tolled options such as public transportation, the study finds. "These facts imply that that a smaller percentage of the poor than the non-poor are likely to be affected by tolling in the Puget Sound region." The lower number is due, in part, to the fact that fewer poor people have cars: More than 26 percent of low-income households don't have a car, compared to just 5 percent of those in the next income bracket.

Moreover, the study concludes, "Using tolls to finance a project will generally impose fewer costs on the poor than using broad based consumption-oriented taxes such as the gas tax."

Read the whole UW study here .