Local transit officials on Thursday adopted a cost control plan for dozens of bus, highway and rail projects called for under Los Angeles County’s latest sales tax to fund transportation improvements.

The action approved by the Los Angeles County Metropolitan Transportation Authority board, would establish a process for preventing, assessing and mitigating cost increases or funding shortfalls should they occur on Measure R projects.

Board members said they were concerned that potential declines in tax revenue or large cost overruns in the future could threaten Metro’s ability to deliver projects that would be paid for with the half-cent sales tax passed by voters in November 2008. Measure R is projected to generate an estimated $40 billion for transportation improvements across the county in the decades ahead.

“If we are not on our tippy toes closely watching the spending, it could threaten projects 20 or 30 years out,” said Los Angeles County Supervisor Gloria Molina, one of several Metro board members who proposed the plan. “We don’t want a bait and switch. We need to deliver on promises made to the voters.”