THE M4 is creaking under the strain of huge congestion made worse by shopping and residential developments deliberately sited near junctions, experts warned last night.
A transport group representing 10 local authorities from Wales says that both business and residential plans backed by the Government will soon put even more strain on the motorway.
Many sites near the M4 have been identified for development because it is seen as the main artery through South and into West Wales.
But the South East Wales Transport Alliance (Sewta) fears that even if new developments are carefully planned to encourage non-car journeys, traffic on the M4 could grow by a quarter over the next 20 years.
It also says that because of poor public transport links across South Wales, the Welsh Assembly Government is simply encouraging more car use by placing developments near the M4.
Sewta says plans for an International Business Park at Junction 33 of the M4 might look like a good idea on paper - and Enterprise Minister Andrew Davies says the development would enable Wales to compete for "world-class projects" - but the development would increase local car journeys on the motorway - impeding long-distance traffic and harming the economy of southwest Wales, says Sewta.
Transport expert Prof Stuart Cole said yesterday he did not envy ministers who had to balance the opportunity to create thousands of jobs at Junction 33 against the environmental and transport issues.
Colin Eades, Sewta's chair of directorate, said, "The main road system around the site - both M4 and the approach roads to it - is seriously congested, as is obvious to any observer.
"Motorways are designed to be conduits of long-distance traffic.
"These [new developments] would clog up the already congested junctions east and west of J33 and reduce carriageway capacity because of the weaving movements involved in entering and exiting the motorway.
"The development could finish up having a negative rather than positive effect on the Welsh economy."