No doubt the next mayor of Minneapolis will embrace R.T. Rybak's most obvious priorities. Who, after all, could be against safer streets, better outcomes for kids and reasonable (if that's possible) property taxes? But there's one Rybak priority that deserves special attention, because it's too often overlooked: population growth.

If Minneapolis hopes to sustain itself as a quality urban place, it must discover how to grow again. Rybak mentioned it in his 2010 budget speech, and his words bear repeating: "If we want to live in the kind of neighborhoods we want, if we want Minneapolis to be the kind of city that we know it can be, there is one more thing we have to do; we have to grow."

Indeed, older cities have little choice but to try to add taxpayers to help shoulder the extra costs of aging infrastructure and disadvantaged populations, especially with declines in federal and state aid in recent years. It's harder still for Minneapolis, which lost a third of its population between 1950 and 1985 as households shrank and middle-class families with children migrated outward (St. Paul's losses were far smaller). More recently, population numbers have inched upward as the housing market recovers and as younger people lean toward urban lifestyles.

The question for the next mayor is: How to take full advantage? How to push Minneapolis' population back up over 400,000 and toward its 1950s peak of 522,000? How to learn from cities like Denver, Seattle and Portland, each of which has recorded spectacular infill population gains in recent years?

One of the best ways is to convert the single-family homes that still line many busy streets into multifamily housing with commercial space at street level. That's what Seattle did along many of its bus routes, a strategy that helped add more than 100,000 people in less than two decades.

The new city-county strategy for Penn Avenue North should be seen as a step in that direction. Hennepin County Board Chairman Mike Opat is right to say that county thoroughfares like Penn should be "amenities, not simply raceways," meaning that they should be transformed into destinations for neighbors, not just kept as pass-throughs for drivers trying to get quickly from one part of the city to another.

These busy streets should be places for small-business job creation, economic recovery, beautification and, yes, population growth. Focusing urban energy on main streets will preserve and strengthen the nearby single-family-home neighborhoods that Minneapolis cherishes. That's what happened when the county rebuilt East Lake Street to help immigrant businesses establish a stronger foothold.

While Penn is more residential in character, the results could be similar if neighbors welcome the zoning changes needed to allow some segments of Penn -- most of them between Olson Hwy. and Dowling Avenue -- to become mixed-use community assets that combine multifamily housing and small-scale job generation.

The North Side, with its low-income, high-crime profile, has been the city's toughest redevelopment challenge. But the Bottineau light-rail corridor that parallels Penn Avenue on the west offers an opportunity to remake parts of Penn in a positive way, not only as a transportation link but also as a livable thoroughfare that helps restore Minneapolis' critical mass and population base.