BMA Cymru Wales hopes the move will reduce the risk of injury and promote physical activity in schoolchildren.
BMA Welsh secretary Dr Richard Lewis said the 'safe route to school' initiative had saved lives but did not go far enough.
'The Welsh Assembly government and local authorities in Wales need to go much further to save even more children from needless injuries and deaths.
'Extending 20mph zones to all walk-to-school routes will help save Welsh lives,' he said.
Lee Waters, director of Sustrans, a charity working to reduce road traffic, said: 'Traffic moving at lower speeds provides an environment that will encourage people to walk and cycle.'
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