For the last five years the road near Llandrindod Wells lake has been closed at night for three weeks to help hundreds of creatures looking for love.
Powys council said the tunnels would protect amphibians from steep kerbs, cars and predators.
A roadside ditch has also been built to guide the animals to the new tunnels.
In previous years, volunteers from Radnorshire Wildlife Trust have carried toads, frogs and newts in buckets because many were unable to climb the kerbs.
The tunnels have been built to the north-east of the lake along with the 400m ditch, which runs alongside the road.
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