What would a 60mph motorway speed limit do for NOx emissions?

There is, however, some previous research, so let’s evaluate.

In 2011, the European Environment Agency (EEA) published a report on the subject.

This study documented a detailed simulation into the effects of dropping motorway speed limits from 120 to 110km/h (74.5 to 68.4mph) for Euro 4-compliant petrol and diesel cars with engines between 1.4 and 2.0 litres in capacity.

With smooth driving and complete compliance with speed limits, the diesel car exhibited a 12% reduction in fuel use and the petrol car an 18% reduction. This is because there is less wind resistance, so the car needs less relative power to propel itself. However, in more realistic driving, including some periods of speeding, this came down to a mere 2% and 3% respectively.

There was also a reduction in nearly all pollutant emissions, and especially NOx and particulate matter (PM) for the diesel – NOx by more than 20%, PM by around 10%.

Contrarily, NOx would actually increase for petrol cars, but these emit 29 times less of the pollutant at 62mph, so the overall effect of the new speed limit would be positive in terms of NOx.

The report concludes by saying: “Decreasing car passenger speed limits in motorways could lead to substantial benefits.”

Those reductions don’t sound like a lot, but they do when you consider than cars covered 49 billion motorway miles in 2015, and around half of the vehicles contributing to that were diesels.

Or that statistics from the Department for Environment, Food and Rural Affairs (DEFRA) reveal that, in that same year, of the 918,300 tonnes of NOx emitted in the UK, 34% was from road transport.

Those statistics show that progress has been made, however. Total NOx emissions in the UK have dropped from 2.97 million tonnes in 1970 to just 920,000 today, in part due to cleaner engine technology.

Longer journeys

A 10mph speed reduction over an entire journey sounds a lot. Indeed, if you were to drive from London to Birmingham, it would take 20 minutes longer at 60mph than 70mph.

Variable speed limits have been proven to slightly increase traffic flow - for example, by up to 9% on the M25.

But when the road is clear, or on roads with a fixed speed limit, journeys would take longer - not helpful for either people's everyday lives or the UK economy, which is already predicted to lose out on £307 billion between 2013 and 2030 due to congestion.

Not a long-term fix

As much as I hate myself for writing this, bringing the speed limit down does seem to be a fairly good idea - but it just seems a bit myopic.

Sure, for now, when diesel cars still make up around half of road traffic, it's beneficial towards meeting our short-term NOx targets that we're currently missing. But, if this lowering does happen, the Government needs to simultaneously focus on heavily pushing both the sale of and construction of infrastructure for electric vehicles. Its proposed diesel scrappage scheme is a good start.

The closure of many coal-fuelled power stations, to be replaced by renewable energy and natural gas ones between 2012 and 2015, decreased the UK's total NOx emissions by 16%. Imagine if the same were done on our roads and, if that trend for the production of electricity to charge AFVs continues, how quickly NOx pollution could be reduced. Remember, of course, that EVs produce no tailpipe emissions.

And, if the move from individual car ownership to 'mobility sharing' that so many car manufacturers are predicting does indeed occur, congestion could also be reduced, boosting the UK economy.