New York City has a lot of tunnels, and a lot of cars drive through those tunnels everyday, releasing all sorts of particulates out of their tailpipes that eventually stick to the walls of said tunnels. To avoid too much buildup, the tunnel walls and roof are cleaned, and here’s how they do that.

For example, the Brooklyn-Battery Tunnel that connects lower Manhattan to Brooklyn under the mouth of the East River is 9,117 feet long, or roughly 1.73 miles of tubular roadway for cars. That’s a lot of tiled walls and ceiling to be cleaned by hand, which is why they don’t clean it by hand.

Instead, New York and cities across the world clean tunnels using big special trucks. Most of these trucks use a combination of high pressure water streams and giant spinning brushes, like you’d find in a drive-thru car wash, to spray and scrub the walls of the tunnel.

This video from the Colorado Department of Transportation shows one of these tunnel-cleaning trucks in action, with narration:

The person talking describes a jet of hot water spraying ahead of the brush, followed by a rinsing stream behind the brush. A second video from the Colorado Department of Transportation shows the side view of this truck, with its giant brush arm extending up to the ceiling of the tunnel, with a trailer carrying a tank of liquid being pulled by the truck and spraying the rinsing stream:

Other trucks have rotating and extending arms for cleaning the tunnel walls. Some have just one brush, others, like this one highlighted by Tech Insider, have multiple which can be adapted for different tunnel shapes:

This one in particular doesn’t use any sort of soap or detergent to avoid contaminated water runoff impacting the environment. Instead, it relies on high pressure water alone.

I love tunnels, but I know a lot of people that are very uncomfortable being underground. To me, it’s only a comfort to drive through a clean tunnel. If the walls are covered in dark buildup, not only does it make me think of all the shit I breathe in on a daily basis, but it also would make me feel like the tunnel wasn’t very well maintained, and that maybe I shouldn’t trust whoever is responsible for making sure the tunnel I’m trusting with my life is in good condition.

Most tunnels feature bright-colored walls and ceilings, often white tile, to limit the darkness of, you know, being underground. But since the particles that get pushed out of a car’s exhaust are dark, it’s going to build up and become very obvious very quickly. Hence the need for cleaning.

Anyway, these are things I think about driving around New York. Operating a tunnel cleaning truck seems like a calm and satisfying job, if I ever need alternate career plans. Good to know.