By any measurement, $15 billion is a lot of money, especially when we’re talking about taxpayer money.

Earlier this year, the Legislature authorized Sound Transit to seek voter approval to collect about $15 billion over a 15-year period to fund the next phase of construction of its Link light rail system, dubbed ST3. Sound Transit, under the direction of a board of public officials, including three from the Snohomish County region, has gathered together a list of potential projects and options within those projects. Among those of greatest interest to voters in Snohomish County, many who will be asked to approve a tax package to provide the funding, is the extension of light rail from Lynnwood’s future terminal to Everett Station.

Sound Transit’s board also is considering five other light rail projects in King and Pierce counties and three bus rapid transit projects.

But even $15 billion won’t be enough to fund all proposed projects. So, before a tax package is presented to the voters next fall, Sound Transit’s board must settle on a list of projects that puts that considerable sum to its best use: moving as many people as possible among homes, jobs, school and other daily destinations.

Everett’s link to Link has to be a priority. More specifically, the northernmost section of a regional light rail system must do more than reach the transit hub at Everett Station; its route must take a jog west from the I-5 corridor to serve the manufacturing center that includes Paine Field, Boeing and a host of other employers large and small.

Economic Alliance Snohomish County, which is working with other city and county public officials on the Everett options, created what it called a “job density heat map,”showing the concentration of jobs between Lynnwood and Everett. The largest, hottest zone is the Paine Field region, the largest manufacturing center in the state.

The cost for the 15-mile route that includes Paine Field is estimated between $4.5 billion and $5 billion, the most expensive of the projects under consideration. An alternative route to Everett Station, more closely following I-5 but bypassing the jobs at Paine Field as well as several densely populated residential areas, is less expensive at about $3 billion.

But a less expensive option that doesn’t serve major job or population centers and carries fewer riders is no bargain.

Among the other projects under consideration are light rail extensions to West Seattle, Ballard and Issaquah, but each are essentially spurs to the main north-south spine that should be ST3’s priority. And none of the spurs show a potential for serving as many as the 58,000 daily riders expected for the Paine Field route. West Seattle would serve an estimated maximum of 50,000 riders; Ballard about 54,000 and Issaquah, 15,000.

Earlier, Snohomish County leaders conceded that extending light rail to the growing campus at Everett Community College and WSU’s University Center, while desirable, would have added too great a cost to the project. Instead, a bus rapid transit line, such as the Swift line now offered along Highway 99 by Community Transit, could serve the college campus and Providence’s Colby campus well.

Supporters of ST3 will have their work ahead of them to win voter approval of a tax package that will include a mix of sales tax, car tab and property tax increases. It’s estimated that the package would add about $200 annually to the taxes paid by a typical adult living in the Sound Transit district, which in Snohomish County will include the cities of Everett, Mukilteo, Mill Creek, Mountlake Terrace, Bothell, Lynnwood and Edmonds.

The support appears to be there. A survey released by Sound Transit early this year showed that 75 percent of Snohomish County voters supported the $15 billion tax request.

But district voters in the region will want to see their money put to the best use: a route that includes service to the jobs and homes surrounding Paine Field.