Tunnels and Viaducts on the D29A Motorway in Italy, by Kilometre Point

The record table

How this archive measures: lengths, bores, dates and ordering

How this table is compiled

The note column places the route in Sicily, and the administrative area is recorded because a section is identified by it. Italian motorway inventories list tunnels and viaducts together in running order along the D29A. Cross-border schemes need two states to fund the same hole at the same time, which is why they take decades.

The kilometre point is recorded for each structure on the D29A, which is how the operator identifies it. A crossing whose two carriageways differ in length is recorded once per direction rather than once per crossing. Ventilation shafts allow a longer bore to be built, and their absence is often what caps a tunnel's length.

Tunnel building in Italy is shaped by geology and settlement pattern as much as by transport policy. Two-way traffic in a single bore is the arrangement every modern regulation tries to avoid and older crossings are full of. Japan's rail network drove long-bore practice decades before its roads did, and its list reflects that order.

Seismic risk changes the lining rather than the alignment: a tunnel is among the safer places to be in an earthquake. Traffic figures are deliberately excluded, since they date far faster than the structures themselves. The note column is often the most useful part of a row, recording tube counts, gradients, closures and reconstruction dates.

Transverse ventilation gives every section its own supply and extract, which is why the long two-lane crossings have a false ceiling. The first railway tunnels were driven by hand and gunpowder, and their alignments still carry the marks of both. Some entries carry a range instead of a single value where the two carriageways differ.

Reading the figures

Norway builds more subsea road tunnels than anywhere else because its coast is deep, its islands are populated and a bridge would not reach. The oldest crossings still in service were built before any of the design codes that now govern their maintenance. A record is a statement about what has been published, not about what exists.

Overburden decides whether a bore is driven or built in a trench, and the switch usually happens within a few hundred metres of a portal. Iceland's tunnels are short, recent and almost all built to replace a road that closed in winter. Two headings driven towards each other halve the programme and double the survey problem.

Motorway tunnels are usually twinned, and the second bore is frequently built decades after the first. Tunnels built for one purpose are regularly repurposed for another, and a disused rail bore is the commonest cycleway in some countries. A projected opening date is a forecast, and on large schemes forecasts have historically slipped by years.

Wartime damage closed several crossings for years, and a few were never reopened to their original use. Gradient limits are the main reason rail tunnels run longer than road tunnels on the same crossing. Sequential excavation takes the face in slices, which is slower per metre and far more tolerant of ground that changes.

Two administrations measuring the same crossing will agree on the rock and differ on the approaches. Rack railways climb where adhesion alone cannot, and their tunnels are short, steep and unlike anything on a main line. A tunnel's gradient is often the more interesting figure, particularly on subsea and base tunnels.