Fast-growing cities in western Ada County continue to get a larger share of recent roadway spending, according to Ada County Highway District’s latest spending-and-revenue report released Friday.

But the trend is about to turn: Boise, which has slightly more than half of the county’s population, stands to get $6 of every $10 ACHD expects to spend on construction through 2020, the study shows.

ACHD analyzed its revenue collections and spending within local government boundaries from 2001 through 2012.

The study, which encompasses $814 million in spending over a 12-year period, is based on audited spending reports ACHD files annually with the state.

The countywide highway district is responsible for all non-state roads in Ada County and each of its six cities — Boise, Eagle, Garden City, Kuna, Meridian and Star.

Among the cities, some inequity can occur in any given year or if one city has a particularly large project, according to the study. For example, Meridian’s $18 million split corridor project and Boise's $21 million East ParkCenter bridge skewed the numbers in each city’s favor respectively.

Growth patterns, too, can cause an inequity, as ACHD must build new or expand existing roads to accommodate new residential or commercial development.

“During the recession, most of the growth we’ve had has been on the west side,” said Sara Baker, ACHD Commission president. “But things are now turning around. Starting this year, Boise City will receive the lion’s share of the spending as we address some long-standing needs.”

One of those needs is the 30th Street extension, a project that broke ground on Wednesday and will be completed in October. The new road will connect State Street to Fairview Avenue, offering a quicker option for getting to downtown Boise and completing a vision for the area dating back to the 1960s. The road, which will cost an estimated $4.5 million to construct and $9.1 million in total, helps fulfill Boise’s aim to revitalize the western edge of the city center.

According to the revenue-and-spending study, in 2012, Boise generated 43 percent of ACHD’s revenue and received 37 percent of its revenue. Meridian generated 21 percent of the highway district’s revenue and received 28 percent of its spending.

But over the 10-year period, the study found the revenue and spending is more equitable with Boise generating 45 percent and receiving 42 percent; Meridian generated 18 percent and received 23 percent.

ACHD is the state’s only countywide highway district.

Ada County voters created ACHD in 1971 in response to frustration with roads being built and maintained by separate city and county street departments.

Today, ACHD maintains 2,240 miles of roads (state highways and the interstate are the responsibility of the Idaho Transportation Department) with an $89.2 million annual budget and about 300 employees.

Property taxes represent the largest revenue source for ACHD ($37.1 million) followed by state gas taxes ($20 million), impact fees ($9 million) and vehicle registration fees ($8.8 million).