Ministers have failed to get to grips with traffic congestion according to a report on transport policy by the spending watchdog, Audit Scotland.
Ministers had pledged to stabilise road congestion by 2021
The wide-ranging review also said £1.5bn was needed to tackle a backlog in local road repairs and that cash for concessionary travel could run out.
However, it said the Scottish Executive performed well against most targets, although more monitoring was needed.
Transport Minister Tavish Scott said he felt the report was "very fair".
Ministers have said they are planning to tackle bottle necks and offer more public transport.
Auditor General for Scotland Robert Black said: "Investment in transport is significant and growing.
"Given the scale of the spending and the importance of a sound transport strategy for the economy of Scotland, and also for the environment and quality of life, I am encouraging the executive to improve its monitoring and reporting of what is being delivered through the transport programme."
The report found that an executive plan to stabilise congestion at 2001 levels by 2021 looked likely to fail - a fact which has already been admitted by ministers.
The total volume of road traffic on Scotland's roads in 2005 was 43 billion vehicle kilometres, 19% more than in 1994.
Latest forecasts estimate the figure is likely to grow by 27% by 2021 and an executive report earlier this year found little "buy-in" from local councils to the concept of traffic reduction.
The report said: "The executive is now considering whether to replace its traffic stabilisation aspiration with targets which better focus on the transport outcomes it wishes to see."
It found that the use of bus services had increased by 23 million passengers since 1999/2000 and satisfaction among travellers was high.
However, free bus travel for older people and people with disabilities, introduced in April this year, prompted financial concerns.
The report stated: "Higher than expected usage of free travel may exhaust the new scheme's budget."
However, the executive said it believed the risk was minimal.
The report also found that the number of rail journeys on ScotRail services had increased by almost half over the past decade, reaching 75.1 million in 2005/06.
Mr Scott told BBC Radio's Good Morning Scotland programme that the executive's National Transport Strategy, which is being published later this year, would address the main concerns.
Mr Scott said investment in public transport was essential
He said: "If we want to spend more money on public transport we need to try and hold our spending on trunk and local road maintenance.
"But I accept there will always be motorists talking about congestion, potholes and delays.
"We will never have a perfect system but we do have to provide increasingly for public transport because we have to provide choice for the motorist."
Mr Scott added that the executive had spent £335m on trunk road maintenance in 2006-2007 compared with just £211m in 2002-2003.
He accepted there was still a backlog but defended the executive's allocation of funding.
"If we are serious about climate change and tackling emissions then it's the government's job to put money into public transport," said Mr Scott.
Scottish National Party transport spokesman Fergus Ewing said the report showed the Labour/Lib Dem administration had failed to spend its transport budget effectively.
"The Lib-Lab government should be establishing more and larger park-and-ride schemes and proactively promoting home-working and flexi-working to give people the chance to avoid the rush hour," he said.
Green MSP Mark Ballard said: "Instead of investing in massive road projects like the M74 and Aberdeen bypass, we should be investing in providing transport alternatives to reduce car use."
A Cosla spokesman said the £1.5bn road bill was a priority and it would continue to press the executive for increased funding to address the problem in the upcoming spending review.
Meanwhile, CBI Scotland insisted the M74 must be completed, while Friends of the Earth said an incomplete picture of any benefits from the new road meant it should be ditched.