The RAC Foundation insists it is not against rail travel
The government is wrongly prioritising investment in rail over roads, despite 92% of all passenger journeys in the UK being made by car, the RAC has said.
According to the motoring body, "the road network is the true provider of public transport".
It said focusing on roads, not rail, would bring much higher rates of return for every £1 of public money spent.
The Department for Transport (DfT) said value for money was "always central to any funding decisions".
More than £5bn of public money was invested in rail in 2006/07 compared with £4.8bn on roads.
Some £15bn has been earmarked for rail over the next five years, compared with £6bn to improve the strategic road network in England.
'Not against rail'
The DfT's public service agreement requires it to prioritise spending in order to get the highest rate of return for every £1 of public money spent.
The RAC Foundation said it wanted to know "why this clear procedure is being ignored by ministers".
It quoted figures from the 2006 government-commissioned Eddington Transport Study, which showed the average benefit-cost ratios (BCR) for transport schemes.
Highways Agency roads had a much higher BCR of 4.66, compared with 2.83 for heavy rail schemes and 2.14 for light rail schemes.
The RAC said that of 35 rail expansion schemes suggested last week by the Association of Train Operating Companies, just two had a BCR above 2.0.
Foundation director Prof Stephen Glaister said: "The foundation is not against rail travel, yet the figures are clear.
"Most people use the roads to get about and road schemes tend to offer the best value for money. The road network is the true provider of public transport.
"When government funds are in such short supply, surely ministers are duty bound to abide by the Treasury's own rules and extract the maximum value for money from every £1 they collect from hard-pressed taxpayers?"
A DfT spokesman said: "Value for money for the taxpayer is always central to any road or rail funding decisions the government makes.
"The investment the government has made to date in rail is reflected by increased passenger volumes of 40% over the last decade.
"But we also recognise the importance of roads and are committed to tackling congestion and providing a safe, reliable and sustainable road network for the 21st Century."