Sustrans Cymru - an organisation that promotes greener transport - has condemned the potential M4 Toll as "poor value for money".
A spokesman said that a "more cost effective way to reduce congestion would be to invest in shifting these journeys to public transport."
"The Welsh Government acknowledges that 40% of journeys on this stretch of the M4 are local trips of under 20 miles," he added.
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Mladic, now 75, was the military commander of Bosnian Serb forces in the 1990s, who fought against the Bosnian Croat and Bosniak armies.
Weather: Heavy rain and strong winds in the north
His representatives said Cassidy, who was hospitalised last week, died "surrounded by those he loved."