If your Friday night involves crossing the city, the road you would normally take underneath it is closed. Main Roads WA confirmed early this morning that the Northbridge Tunnel will be shut in both directions from 8.30pm on Friday 18 September until 5am on Saturday, a full overnight closure for maintenance.

The agency announced it just after 5.30am on its Perth traffic account, and singled out one of the jobs on the night shift's list.

"One job that needs to be conducted tonight will be cleaning of the tunnel walls. This is to maintain visibility, reflectivity, and lighting levels for road safety." — Main Roads WA

What is closed, and for how long

This is a full closure, not a lane closure. Both the eastbound and westbound carriageways of the Graham Farmer Freeway's tunnel section are out of action for eight and a half hours, from 8.30pm Friday to 5am Saturday morning.

The tunnelled section runs for 1.6 kilometres, entering the ground at Fitzgerald Street in Northbridge and surfacing again at Lord Street. Everything either side of those portals — the run west to the Mitchell Freeway, and the run east over the Windan Bridge towards Burswood — is a different matter, but the through route is broken for the night.

Main Roads has not published a signposted detour, and its advice is the short version it usually gives: be aware, and plan ahead. Its live travel map is the place to check on the night, because overnight works occasionally finish early or run long.

Why crews wash a tunnel

Scrubbing walls sounds like housekeeping. It isn't, quite. A road tunnel is lit artificially every hour of the day, and a large share of the light drivers actually use is light that has bounced off the tunnel's wall panels rather than come straight down from the fittings. Exhaust film, brake dust and road grime build up on those panels and quietly eat that reflected light, so the same lamps deliver a dimmer tunnel year after year.

Washing the walls restores the reflectivity, which is precisely the word Main Roads used. The alternative — leaving the grime and turning the lights up — costs more power and does less for what a driver can actually see. Crews also get the tunnel to themselves for the other work that cannot be done with traffic running through it: checking ventilation, signage, drainage and the emergency systems.

Getting across town without it

The Graham Farmer Freeway exists to take east–west traffic off the surface streets of the CBD and Northbridge, so when it shuts, that traffic goes back where it used to go: Newcastle Street and the streets around it, Wellington Street, Riverside Drive and the Causeway. Late on a Friday that is a manageable load rather than a crisis, but it is a slower trip than the one you are used to, and it puts cars back onto streets full of people walking between venues.

It is also a Friday night with a Fremantle preliminary final on it, played on the other side of the country and finishing well before the closure begins. Expect the usual post-game traffic around pubs and the entertainment precinct — with one of the city's main bypasses unavailable underneath it.

A 26-year-old shortcut most of Perth takes for granted

The tunnel is the centrepiece of the 6.5-kilometre Graham Farmer Freeway, which links the Mitchell Freeway at West Perth to Great Eastern Highway and Orrong Road at Burswood. It is named after Polly Farmer, and it opened to traffic on 24 April 2000 after being built the hard way — top-down, with the roof and walls poured first and the middle dug out afterwards, which tore up Northbridge for years.

It was never built with much room to spare. In 2013 the emergency stopping lane in each direction was converted into a third traffic lane to keep up with demand, which is part of the reason a closure is the only realistic way to service it properly. There is nowhere for crews to stand while the traffic keeps moving.

Barring a change, the tunnel reopens at 5am on Saturday.

Photo: Steelkamp via Wikimedia Commons (CC0). The image shows the Northbridge Tunnel's western entrance.