IF public policy were based just on what we know works, universal pre-kindergarten education would already be the law of the land. The public duty to educate children would not, magically, kick in at age 5 for kindergarten. High-quality preschool would be a foundation for school readiness, leveling academic disparities across race and income lines.

But such a utopia is not to be found. The Washington Legislature is moving, slowly, in that direction; Congress, less so. Cities have begun to redefine the public duty to the tiniest of students. That is why the City of Seattle’s proposal for a universal, high-quality preschool experiment seems promising.

The Seattle preschool proposal, cultivated over the past year by City Council President Tim Burgess, imports proven models from Tulsa, Okla., Boston and elsewhere.

Envision teachers with bachelor’s degrees providing six hours a day of curriculum-based instruction to 3- and 4-year-olds, at a tuition cost that is far less than most middle-class parents now pay for lower-quality child care. For a family of four making $95,000 a year, the annual tuition would be $2,100.

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The program would be funded with a four-year, $58 million property-tax levy, which the City Council is likely to put on the fall ballot. Even at that size, the Seattle preschool levy requires an incremental start — growing from about 300 children in 2015-2016 up to 2,000 in 2018-2019.

Once fully implemented, the Seattle preschool proposal would cover about one-sixth of Seattle’s 3- and 4-year-olds. While not preschool for all, it is wise to start slowly. Many of the benefits of preschool are based on quality, which is complicated to achieve. Cutting corners — such as on minimum education levels of teachers — negates the gains.

Done well, high-quality preschool provides tremendous civic good. A Rutgers University study last year found that 20 to 40 percent of the achievement gap between white and minority students was closed by two years in high-quality preschool instruction in Abbott, N.J., with the best results in math and science.

A much longer-term study, involving a superb preschool model pioneered in Ypsilanti, Mich., found a cost-benefit ratio of $17 saved for every $1 spent by the time the participants turned 40.

In the coming months, the council should fully vet this proposal, gauging the interest to participate from existing child-care centers and preschools. It should also resist pressure from a group called Yes for Early Success, which is backed by the SEIU local representing in-home child-care workers, to significantly tinker with the proposal.

Adding another levy to Seattle’s bulging menu of pending requests — including the proposed doubling, to $48 million, of the local parks levy, a $45 million Metro Transit package and the uncounted costs of a $15 minimum wage — creates an unnecessarily strong headwind for the Seattle preschool package.

Creating universal, high-quality preschool is an opportunity for Seattle to walk its talk about progressive values. It deserves a spot at the top of the list.

Editorial board members are editorial page editor Kate Riley, Frank A. Blethen, Ryan Blethen, Sharon Pian Chan, Lance Dickie, Jonathan Martin, Erik Smith, Thanh Tan, William K. Blethen (emeritus) and Robert C. Blethen (emeritus).