DENVER – A new law to change the way school boards negotiate with teacher unions was the only statewide Colorado ballot measure approved by voters Tuesday.

And it’s not clear how the little-noticed Proposition 104 will change the way teachers and school districts negotiate.

The measure won with nearly 70 percent of the vote, the only one of four ballot measures to be approved by voters.

It directs Colorado to join a handful of states in requiring school boards to allow the public to see negotiations on collective bargaining agreements, or union contracts.

The measure does not apply to other governmental entities that negotiate with unions.

Only about a fourth of Colorado’s school districts have collective bargaining agreements, but those districts are larger and account for three-quarters of the state’s public-school students.

Some large Colorado school districts, including Jefferson County and the Poudre School District in Fort Collins, already negotiate in public.

The measure was backed by the Independence Institute, a conservative think tank based in Denver. Supporters said it could help taxpayers understand a budget item that in some cases makes up the bulk of a school board’s budget.

“It brings more transparency to the process,” said Ben DeGrow, the group’s education analyst.

The proposal was opposed by the state’s largest teachers’ union, the Colorado Education Association, and by the Colorado Association of School Executives.

CEA spokesman Mike Wetzel called the measure vague.

“We’re all about transparency in education, but the language is very broad. It doesn’t really spell out how this will work,” Wetzel said.

Proposition 104 is a new law, not a constitutional amendment. So state lawmakers are free to try to make changes.

Wetzel said it’s too soon to say whether his group would push for legislative changes to clarify the new requirement.

“It brings up more questions than answers, so we’ll have to see how it plays out,” Wetzel said.