The Victorian Auditor-General's Office assessed the installation of wire rope barriers on 20 high-risk rural roads across the state, under the $340 million 'Top 20' program, and the roles of VicRoads and the Transport Accident Commission.

One of those roads is the Calder Freeway between Bendigo and Keilor Park.

VAGO found installation was likely to go over budget by 22 per cent, and the barriers - while saving lives and reducing serious injury - might not work as well as expected.

The state government's Towards Zero strategy says flexible barriers can reduce fatalities and serious injuries from head-on and run-off-road crashes by up to 85 per cent.

But analysis of 18 completed projects found barriers, along with rumble strips and wide centre lines, have most likely reduced fatalities and serious injuries on these sections of road by 46.5 per cent.

"If this result persists, then the projects will not achieve their expected benefits and the Top 20 Program will be less cost effective than intended," the VAGO report said.

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VAGO said VicRoads was likely to go over budget because it had not adequately planned the installation of flexible barriers.

The report also said VicRoads had failed to properly maintain and monitor the barriers, increasing the risk they would not work properly.

The audit found VicRoads had not kept adequate records of the project, which would hamper the ability to evaluate the effectiveness of the program.

The report said VicRoads and TAC did not prepare a business plan, which would have allowed for a better-informed decision about the investment in the program.

VAGO also noted that VicRoads relied on crash data for all vehicle types, of which passenger vehicles make up the majority, but its plans and proposals contained no information about the effectiveness of flexible barriers for other types of road users, such as those in heavy vehicles and motorcyclists.

The audit revealed stakeholder engagement was not completed before construction began, so two projects - including the barriers on the Calder Freeway - had to be modified afterwards to address concerns.

It was also found the program was running overtime.

VicRoads now has plans to conduct an evaluation, but does not expect to finish it for another six years.

Regional Roads Victoria's chief regional roads officer, Paul Northey, said there was no price that could be put on the lives saved by wire rope barriers.

"We are encouraged by the crash data that support these stories - fewer people are dying and being seriously injured on the roads where we've installed flexible barriers," Mr Northey said.

VAGO made eight recommendations to VicRoads, including developing business cases for major infrastructure developments, forming crash reduction factors supported by multiple peer-reviewed sources, and developing maintenance standards for wire rope barriers.

It was also recommended the TAC require a business plan from VicRoads before approving funding for major developments, and develop a reporting framework for the Top 20 program.

VicRoads and the TAC have accepted all recommendations and developed action plans to address them.

"We're pleased that the Victorian Auditor-General's Office report acknowledges that safety barriers save lives," a government spokesperson said.

"We've accepted or already acted on all recommendations from the report."

Mr Northey said Regional Roads Victoria was delivering a range of safety measures in partnership with the TAC.