FORECASTS for growth in traffic on the M4 are too high, a transport expert told AMs yesterday.

Prof Phil Goodwin, professor of transport at the University of the West of England, criticised work on the M4 consultation saying it was simply not credible to presume traffic would continue to grow forever unchallenged.

He said elaborate coloured graphs in a report on the project would amount to “colouring in” if forecasts are unreliable.

The transport professor was speaking at an inquiry by the Assembly’s environment and sustainability committee, which is looking into plans for the M4 Relief Road.

The consultation paper for the M4 Relief Road says that by 2035 the motorway around Newport will be heavily congested, and argues a new motorway could help to significantly reduce such problems.

One graph suggests that forecast trips in Newport and South East Wales will grow continuously from now until 2030.

“I think the growth forecasts are wrong, I think they are too high,” Prof Goodwin argued.

“We will not see traffic growth rates in the next ten or 50 years’ analogous to traffic growth rates we saw during the 1980s. I think that is a transition which took place not recently, but in the mid-1990s.”

Prof Goodwin said: “(The appraisal) is completely dependent on the confidence you place in the Department for Transport regional trip rate forecast and on the implication that traffic growth in future will follow this track for the next quarter of a century and a beyond.

“Actually, the levelling off of traffic growth started ten years before the recession. It’s simply not credible to presume a single future of continued uninterrupted traffic growth forever”

He said the world had changed and that “planning now has to be robust for any levels of traffic growth – or none at all, or even a decline – on low traffic roads, not only continued uninterrupted fast growth into the future.”

Prof Goodwin argued that “policies and projects which are geared to recognising or achieving lower road traffic growth, and are more robust to grievous forecasting errors, are likely to perform very much better than implied in the rather elaborate coloured pictures in the report.”

The West Country-based expert said he had “great pleasure” in looking at the coloured graphs in the report, “but if the forecasts are unreliable (they) are basically just colouring in.”

He held out that it was a possibility the forecast could come to pass, but said if it did the planned expansion of road capacity being considered is not nearly enough to improve traffic conditions.