Underwater tunnel

Ferries also usually use fossil fuels emitting greenhouse gases in the process while most railway tunnels are electrified.

On the other hand, the free surface effect is a significant safety risk for RORO ferries as seen in the sinking of MS Estonia.

Tunnels which exclude dangerous, combustible freights and the fuel or lithium-ion batteries carried aboard motorcars can significantly reduce fire risk.

[citation needed] This may mean that over short distances bridges may be preferred rather than tunnels (for example Dartford Crossing).

Also tunnels don't have the flexibility to be deployed over different routes as transport demand changes over time.

Fixed infrastructure such as bridges or tunnels represent a much more concrete commitment to sustained service.