When a road passes through an enclosed area you could say it’s going through a tunnel, but what about if it’s under a short bridge or if there are windows on one side? A tunnel has a very specific definition in roading: it has to be at least 80m long. Tunnels 80-120m long are ‘short tunnels’ while tunnels longer than that are ‘long tunnels’. This means that when you pass under a motorway, even though it might be 40m, that’s an underpass, not a tunnel.
However, there’s nothing to stop people or local authorities calling anything a ‘tunnel’ even if it’s less than 80m. Australia has a few long(ish) tunnels, e.g. the M5 which is 4km, but that pales into insignificance when you see tunnels from Scandinavia and Europe, the longest of which are more than 20km; in Europe, tunnels of 500m are considered long tunnels.
Tunnel length is often considered in relation to the risks it poses to road users when passing through. The safest tunnels are single-direction tunnels where traffic flow is separated, but not all are like this.
Tunnels are built as a shortcut through steep terrain or to allow buildings or other infrastructure to exist on top of them, e.g. the Tugun Bypass (M1) where the extension to the Gold Coast airport runway creates a 334m tunnel.
There’s an elevated risk of an accident near the beginning and end of tunnels so while a tunnel might be convenient, it’s not necessarily as safe as a regular road. Tunnels can be created using ‘cut and cover’. This means a cutting is made and then a roof is built on top. Of course, if the roof isn’t built, it remains as a cutting, not a tunnel. Tunnels can also be created by boring a hole.