The world may have possibly found a solution to climate change in an unexpected place: the west of England metro mayoral race. It seems that Lesley Mansell, the Labour candidate, let slip what could be a universally applicable method for stopping motorway emissions.

“You could potentially put the M32 underground and then reuse the space on top,” Mansell said at a hustings event held last month by the Bristol Post. “It might seem like a wild idea, but sometimes those off-the-wall ideas can have some positive outcomes. It would be much better if you have got the traffic underground, as we wouldn’t have the carbon going into the atmosphere.”

Of course, it’s a terrible idea. Mansell is possibly correct that the move would create extra space, and she’s possibly correct that wild ideas can work out really well. It’s just that last point where the idea starts to fall apart, as tunnels don’t possess any carbon-destroying abilities.

“[Carbon dioxide] would not be removed by a road tunnel and would consequently emerge into the local atmosphere from the ends of the tunnel,” says Roy M. Harrison, professor at the University of Birmingham’s School of Geography, Earth & Environmental Sciences. There  are many actions we can take to fight climate change, but burying one of the country’s shortest motorways is not one of them.

The M32. Image: Open Street Map.

The motorway is something of a local frustration. Built in 1966, it stretches just 4.4 miles from Bristol’s city centre to the M4 at junction 19. Bristol City Council has tried before to convert it to an A-road, but to no avail. The M32 even has its own entry on PatheticMotorways.org, which documents Britain’s most ridiculous motorways.

Oddly enough, the city council has not ruled out Mansell’s idea. A spokesperson told the Bristol Post that a new congestion task group will be launched soon, aimed at exploring ways of how to improve transport in the local area.

“No decisions or firm plans have been made at this very early stage,” the spokesperson said.

If the task group opts to build a tunnel, for whatever reason, it wouldn’t stop pollution per se, but it would help shape the escaping pollutants. Ventilation shafts would be needed to maintain a somewhat breathable quality of air inside the tunnel. Because of this, building a tunnel would essentially concentrate emissions around a set number of extraction points, rather than allowing them to freely disperse into the atmosphere.

“You could probably build a tunnel with ventilation shafts that suck the air out and have it scrubbed of pollutants,” says James Lee, professor at the University of York’s National Centre for Atmospheric Science. “But I would imagine this would be a very expensive option.”

A tunnel could, in limited circumstances, stop certain emissions from entering the atmosphere. That’s because particles are created from three different sources on cars: combustion, brake wear, and tyre wear. The tunnel would not stop the first kind from leaving, due to their gaseous nature; but the other two could stick to the tunnel walls.

“The reduction in particle loading because of adhesion to the tunnel surface wouldn’t be a particularly effective control technology,” said James Longhurst, professor of environmental science at the University of the West of England.

But it would be wrong to say that city planners never consider tunnels as a method of maintaining acceptable air quality levels. If building a new road would otherwise push an area past national or European requirements for individual pollutants, placing it underground allows designers to concentrate emissions through the shafts, towards areas where the quality is less of a concern.

This is less of an issue with the M32. Bristol’s Air Quality Management Area, a designated zone that local authorities monitor to ensure they meet pollutant targets, only covers a small section of the motorway. The city council is more concerned about the amount of pollution around the city centre, so using a tunnel to manage air quality would do little to help in the areas where it’s needed most.

“The idea of a tunnel is not something one should remove from one’s thinking,” says Longhurst. “It’s just that in the case of the M32, the cost would be very substantial.”

So yes, in very limited circumstances a tunnel can help with air quality management – but no, they don’t stop carbon emissions.

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