The route would have bypassed a busy section of the M4 around Newport
More than £15m was spent on research for a proposed M4 relief road around Newport before it was scrapped as "unaffordable", say the Conservatives.
Deputy First Minister Ieuan Wyn Jones axed the planned £1bn route in July, saying its estimated costs had soared.
But the Conservatives said the millions spent showed the previous commitment to the scheme and it was now a "waste".
The assembly government said much of the feasibility work could be used in alternative schemes planned for the M4.
Mr Jones, who also holds the transport portfolio, announced the proposed 14 mile (22.5km) road from Magor to Castleton in September 2007, saying it could open in 2013.
The announcement was against the background of two fatal crashes in previous months which caused gridlock to the south Wales corridor, prompting business leaders to say traffic congestion was hitting the area's economy.
But in July this year, the proposed M4 relief road was dropped from the assembly government's five-year transport plan.
The figures obtained by South East Wales AM William Graham showed £15.5m was spent on the design development of the Magor to Castleton route between 1998 and 2008, with around half of that in the previous two years.
Of the £8.4m spent in the previous two years, nearly £700,000 went on surveys and £7.1m to technical advisers.
Mr Graham said: "The proposed M4 relief road was one of two major strategic requirements for the transport network in south east Wales.
"It is clear that even as recently as last year the assembly government was committed to its development and was spending millions on turning it into a reality.
"While we accept that major projects such as this inevitably require careful planning over a number of years £15m has effectively been wasted by the assembly government's decision to abandon it."
He called on Mr Jones to say whether his decision was taken in the weeks or months before July this year.
Mr Graham added: "Or was he being advised to scrap it last year, at the same time as spending on its development was higher than all but one of the previous 10 years?"
The Welsh Assembly Government said: "The money spent from 1998 included a considerable amount of work to determine the feasibility of the scheme.
"Much of this work can now be used in the delivery of alternative schemes along this major route.
"The massive escalation in construction costs for the relief road, from an estimated £340m in 1998 to approximately a billion pounds today, has made this scheme unviable.
"However we will be implementing a series of practical improvements along this main corridor to ensure that businesses and individuals using this route will benefit from improved traffic flow at considerably less cost."