Christmas Island rangers have taken road safety to a whole new level with the construction of a "crab bridge" over the island's busiest road.

In preparation for the annual spawning season, rangers have been setting up barriers along the roadside to prevent the crabs from being crushed under the wheels of cars.

A five-metre-high bridge has also been constructed at one point along the road to help the crabs move across the island and continue their migration.

Christmas Island Tourism Association marketing manager Linda Cash said aside from the practical use, the bridge had also become a tourist attraction.

"Sydney can have its Harbour Bridge, and San Francisco its Golden Gate bridge, but it's our crab bridge which is currently wooing tourists from all over the globe," Ms Cash said.

"The ubiquitous crabs take the most direct route possible from their burrows in the rainforest terraces down to the coast, turning the roads, golf course and beaches into a crimson carpet."

Rangers on Christmas Island have also constructed 31 underpasses across the island to assist the migrating crab masses en route to spawn.

More than 50 million crabs are expected to be involved in the annual migration, predicted by the Tourism Association to take place on January 6.