Expressway inspection: watching a road that cannot stop

Expressway deterioration splits into pavement, structures such as viaducts, bridges and tunnels, ancillary structures such as signs, lighting and noise barriers, and cut and fill slopes. Japan began building expressways in earnest in the 1960s, so some sections are now more than fifty years old. Repeated heavy-truck loading, de-icing salt in winter and water ingress are the three main drivers of deterioration.

In 2012 ceiling panels fell in the Sasago Tunnel on the Chuo Expressway, killing nine people. From July 2014, bridges, tunnels, sign gantries and similar road structures in Japan have required periodic inspection every five years, based on close visual inspection. Expressway operators (e.g. NEXCO East, Central and West, Metropolitan Expressway, Hanshin Expressway) layer daily patrols and their own detailed inspections on top.

The pain particular to expressways is that getting close means closing lanes. Closures are usually short night-time windows, and vehicles crashing into the closure itself are a major hazard for crews. Pavement, by contrast, is mainly measured by road-surface survey vehicles that record cracking, rutting and roughness at traffic speed — a “measure without stopping” system already exists there.

On expressways the drone is a tool for seeing faces invisible from the carriageway without closing the mainline. A realistic split leaves pavement to survey vehicles and gives the drone viaduct soffits and sides, the tops of sign gantries, the back of noise barriers, slopes and the hours after a disaster.

The first rule of flight planning is a route that never passes directly over moving traffic. Approach viaduct soffits from outside the right-of-way or from the local road below, and sign gantries from a closed lane or the shoulder. If crossing is unavoidable, do it inside a window when traffic has been stopped.

The biggest obstacle on an expressway is not the sky but the vehicles and people below. Embankments and viaducts are exposed to wind, and every passing truck sends a gust of turbulent air. In mountain sections, valley winds and the height of tall cuttings come into play too.

• People and vehicles below: a drone falling onto traffic is a serious crash. Settle the no-overflight route, the tie-in with closures and the possible fall zone first. Urban areas & people below
• Wind & turbulence: viaduct decks and embankment shoulders are windy, and trucks and noise barriers create gusts and eddies. Keep margin. Wind & turbulence
• Height: on tall piers and the top of cuttings, the distance and sight line to the pilot get worse. Plan observers and reliable links. Height
• Crowded radio spectrum: urban viaducts sit in busy RF environments. Decide what the aircraft does if command or video link drops (hover or return).

An expressway is a linear asset. Tens of kilometres of images and point clouds are useless unless tied to kilometre posts and structure IDs. AI is good at proposing defects and computing change against the last point cloud. Deciding what to repair first, or whether to close the road, is the road operator’s responsibility.

Structure inspection takes place within the Road Act’s periodic inspection framework (every five years, based on close visual inspection). The February 2019 revision kept close visual inspection as the basis but opened the way to inspection-support technologies such as drones (generally described as allowing methods the inspector judges to give an equivalent diagnosis). Pavement and slopes have their own separate inspection guidelines.

On the flying side, moving traffic is generally treated as people below and overflight is avoided. Specified flights need permission or approval plus a flight-plan notification and flight log, and work with lane closures involves agreement with the road operator and a police road-use permit. In the United States, state DOTs use drone inspection widely while the FHWA sets bridge inspection standards. Compare countries in world regulations.

• Network-wide pavement measurement: for hundreds of kilometres of road surface, survey vehicles at traffic speed are faster and more consistent. Drones suit spot checks.
• Free flight over traffic: avoiding moving vehicles is a given, so closures and timing still matter.
• Sounding, touch and bolt-tightness checks: delamination and loosening are still found by tapping and touching.
• Damage under coatings or inside members: steel thickness and internal cracks need ultrasonic or other NDT.
• Rating and closure decisions: whether to close the road and what to repair first remain human calls.

This is how the Airffic editors picture it. Drone ports sit in the corners of service areas and interchanges. At night, when traffic is light, aircraft fly fixed routes past viaduct soffits, sign posts and slopes — never over a live lane. By morning, only the structures that changed since last time appear on the operator’s screen, tagged with kilometre posts.

On a night of heavy rain, drones check the slopes first and speed up closure decisions. Lane closures happen only to repair or to confirm, not to search. Fewer days with the road shut means, directly, fewer crashes and less congestion.