In a review of major capital projects in Scotland, Audit Scotland examined the £4.7bn in capital spend on roads, railways, prisons and hospitals.
The report suggests that those approving projects must be able to properly scrutinise and challenge proposals for major projects, with clearer information about costs, value for money and timescales.
Robert Black, Auditor General for Scotland, said: "Current and planned major capital works in Scotland make up a large programme by any standards, and it requires the Scottish Government to make very important choices about which projects should go ahead.
"Good decisions can only be made if there is accurate and robust information about the likely costs, benefits and timescales of projects," he said.
They found that only two-fifths of projects were completed within cost estimates on approval. However, at contract award stage, information about costs and timescales was more accurate.
Mr Black said: "It is good news that projects mostly come in close to the costs and deadlines that are set when contracts are being awarded. But there needs to be improvement in the information that is available at the earlier stage when important choices are being made about which projects should be committed."
The report recommends the Scottish Government should report in public on the performance of projects, particularly any reasons for changes to costs and timing and what lessons can be learned from completed projects.
The group say that the proposed 'Scottish Futures Trust' model for funding large infrastructure projects could address many of these issues, with a, "focal point for coordinated public sector infrastructure planning and investment."