The company operating the toll road on the M1 motorway expected that tolls would only ever go up and not down, the Commercial Court has been told.
The court is hearing an action taken by the National Roads Authority against the company operating the toll road.
The NRA claims the company is overcharging motorists.
The authority says the Celtic Roads Group (Dundalk) Ltd is not interpreting the relevant by-laws correctly and stands to make €26,000 a week or €1.39m a year by overcharging motorists.
The NRA says the toll should have been reduced on 1 January this year from €1.90 to €1.80 per car and the court has heard there is no way of repaying motorists who have been overcharged.
The case has implications for other toll roads, as the operators of the M4 toll road, the M8 and N25 roads have also refused to lower their prices.
Mr Justice Peter Kelly was told today that the tolling company is claiming it had a legitimate expectation, including a promise from the former Minister for Transport, Noel Dempsey, that tolls would only go up.
The court heard that Mr Dempsey told the Dáil in June last year that the by-laws provide for an upward only review of tolls.
The NRA says that the by-laws do not make any express reference to an upward only toll system and in fact provide a mechanism to allow for a decrease.
Senior Counsel Shane Murphy said the company was advocating an interpretation of the by-laws which would insulate them from all external realities.
He said that the Minister's statement could not be regarded as a ‘promise’ as it was said after the contract between the NRA and the company was finalised in 2004.
The court has heard that the Consumer Price Index - which is used to calculate the maximum toll allowed - fell in 2009 for the first time in 50 years.
The by-laws allow for a ‘cushion’ of a year - so tolls did not fall.
But the court was told that after the Price Index rose very slightly in 2010, the NRA says the calculation to be applied showed the toll should now decrease.
Mr Justice Peter Kelly agreed that the calculation used to work out the maximum allowable toll was ‘labyrinthine’ and remarked ‘there must be an easier way to do this’.