In theory, Miyong and Sean Kim's Minute Mart should be seeing some foot traffic. The business sits just 500 feet away from the Mount Baker light rail station, which opened in 2009, and stocks any energy drink or snack you could possibly imagine. There's hot food, too. Big signs advertising the Minute Mart's "DELICIOUS DELI" note that Miyong and Sean serve barbecued ribs, chicken, and gizzards.

But instead, the store feels completely isolated from light rail commuters. The Kims took over the business in 2007, right before the financial crash. When the light rail station opened two years later, they were hopeful that it would bring more customers to their small slice of the neighborhood. But according to Miyong, that never happened.

"It didn't help us at all," she said.

The Minute Mart, along with several other businesses along this stretch of Martin Luther King Jr. Way South, sit near tiny, undevelopable plots of land left over from the last light rail expansion. The vacant lots are too small to build anything on, so instead they're just surrounded by chain-link fence. These don't make for a particularly inviting destination once commuters step off the light rail—if they step off at all.

This fall, Seattleites will vote on Sound Transit 3, the biggest regional light rail expansion plan in the area's history. But unlike the Sound Transit 2, the last round of light rail build-out, the new plan invites a radical change. In authorizing Sound Transit 3, the state legislature has asked that the agency buy up more land than it needs in order to build affordable housing near transit stations.

The new Sound Transit 3 requirement is unlike anything that's been attempted in the past. It stipulates that 80 percent of the surplus land Sound Transit acquires to build the new light rail extensions must be offered up first for affordable housing. Whoever develops the plot then must make 80 percent of the units available to people making 80 percent of area median income (AMI) or below. (In King County, 80 percent of AMI means that a family of four would be earning $65,800; a single person's income at 80 percent of AMI would be $46,100.)

Kelly Rider, policy director at the Housing Development Consortium (HDC), characterizes the change like this: "Instead of Sound Transit just going in and saying this is about the miles of rail and how cheaply we can do it, it's 'How do we make sure there's a better community here when we leave it?'"

Sound Transit didn't mean to create vacant, puzzle-piece plots like the ones found around Mount Baker Station. As Transportation Choices Coalition's director of programs Shefali Ranganathan explains it, Sound Transit's directors thought they were doing right by buying just enough land to stage the second build-out of the light rail tracks. "The agency was in financial trouble, so they were acquiring as little land as they had to because they were under serious budget constraints," Ranganathan said. "The unintended consequence is you have these little slivers of land that aren't really developable."

Since then, Sound Transit has evolved its idea of what it means to build out light rail—a necessary development and long overdue. The transit-oriented development policy the Sound Transit board adopted in 2012 aims to increase access to light rail stations and encourage community development around them. That kind of thinking has led to some successes. With the Capitol Hill and Othello light rail stations, for example, Sound Transit encouraged developers to use its surplus land for affordable housing projects. The encouragement worked—more than 150 affordable housing units are planned above Capitol Hill Station, for example—but the developers' decisions weren't in response to any formal housing rule. Nothing required the agency to make its surplus land available to affordable housing developers, and nothing required the developers to actually create affordable housing.

Nothing quite like the proposed "80-80-80" system in Sound Transit 3 has ever existed before, nor does anything like it appear to exist elsewhere. "Los Angeles and Boston have specific requirements around using surplus property for public purposes, specifically for affordable housing," Rider, of the HDC, told me. But "their targets are significantly lower."

Voting on a Sound Transit proposal that encourages the agency to work like an affordable housing advocate also comes at a critical time. More than 125,000 households in King County are already putting more than half their income toward housing alone. And amid the regional housing crisis, low-income communities of color face some of the highest risk of being displaced by light rail development and gentrification.

Fears around displacement made up one of the biggest concerns in a recent Puget Sound Sage and Got Green survey of 175 South Seattle residents. Recent rental trends justify those fears: In Rainier Valley, the average rent for a one-bedroom apartment increased by 8.4 percent in just one year. And while the Puget Sound Sage and Got Green report noted that expanding light rail is central to reducing greenhouse gas emissions from cars, the authors also argued that "reductions are only possible with effective land use and transportation design, coupled with affordable housing."

"One of the costs not factored into affordable housing is the cost of commuting or cost of transportation," Dionne Foster, policy and research analyst at Puget Sound Sage, said. As rents increase and poorer people are forced to the fringes of urban centers, they're often paying more to commute longer distances to work. At the same time, Foster pointed out, they may be driving older models of cars with less fuel efficiency.

That equation changes if you build affordable housing near transit.

"Eighty percent of Sound Transit property could be really big," Foster said. "If we can get Sound Transit to buy the right kind of land for its staging, one model is community land trusts: land that's for affordable housing, owned by the community in perpetuity."