THE case for an M4 relief road could have been based on a false premise as it didn’t take into account the possible impact of a metro system taking traffic off the road, the Senedd heard today.

Questioning transport minister Edwina Hart, Liberal Democrat Eluned Parrott AM said that the planned new metro system of new rail and bus links in South East Wales could result in more than 40,000 extra public transport journeys per day.

But she said: “The Welsh Government’s consultation document on the M4 clearly states traffic growth projections were based on population, households, workforce, employment and car ownership. At no point is the transformational potential of the South Wales Metro considered in that paper.

Ms Parrott AM said the metro’s success could be greater than projected: “We know the opening of the Ebbw Vale line for example exceeded its estimated use by five times. Other studies show that where there is a significant investment in light rail for example there can be a 20 per cent modal shift from car to tram, traffic can be reduced by up to 14 per cent.

"In fact the M4 report specifically states it doesn’t assess the impact of public transport. So which is it?

"Do you believe the metro is going to have this transformational impact and if you do, doesn’t that mean your assumptions for building a new M4 are based on a false premise?”

Edwina Hart AM responded: “We are doing all the necessary work on the metro because I do believe the metro can be transformational. I don’t have a problem with this, this is a twin track approach dealing with public transport investment and also dealing with immediate issues on roads which of course businesses want me to get on with.”

An M4 relief road is planned for the south of Newport, to be open around 2022.

Friends of the Earth Cymru recently launched a bid for a judicial review of the plan.