Calls for 20mph limits to reduce NOx emissions

Calls for 20mph limits to reduce NOx emissions

Calls for 20mph limits to reduce NOx emissions

Campaign groups are calling for lower speed limits in urban areas in a bid to reduce the amount of NOx pollutants being emitted from diesel cars.

Research by Imperial College London found that the average Euro 4 compliant diesel (between 1.4- and 2.0-litres) emits an extra 8.2% of NOx at 30mph – 0.81g/km compared to 0.74g/km.

In 2014, 37% of cars on UK roads were diesels – so reducing speed limits in urban areas could lead to a substantial reduction of NOx emissions.

The research is being highlighted by the 20’s Plenty for US campaign group, which says a blanket reduction of speed limits in urban areas will improve safety as well as improving air quality.

20’s Plenty for Us founder and campaign director, Rod King MBE, said: “A 20mph built up limit simply, immediately and effectively reduces dirty fumes. This government should urgently wake up to the air quality gains from 20mph. Lower speeds give so many road safety, active travel and public health benefits.”

The research also suggest that CO2 emissions from diesel cars in urban areas could be reduced by around 1% by decreasing speed limits in town and city centres to 20mph – but it would have the opposite effect on petrol cars, increasing CO2 emissions from these by around 2%.

As a result, the study found that 100 cars (a representative mix of petrols and diesels) would emit 24,591g/km CO2 at 20mph, compared to 24,309g/km  at 30mph. That’s an increase of 282.5g/km CO2.

3 replies
  1. David
    David says:

    These type of “findings” annoy me.

    The amount of work the engine is doing, rather than the speed at which you drive is what affects the amount of emissions the engine will produce.

    If you drive at 20mph in gear 2 at say 2200 rpm you are going to produce more emissions than moving along at 30 mph at 1200 rpm in gear 4.

    Why can’t you trawl along at 20mph in gear 3 at about 1300 rpm you may say?

    The speed limit being as low as 20mph will mean that naturally each council will install more speed humps or “traffic calming” measures. That in turn will force you to constantly brake and accelerate before and after speed humps (yes even going over a speed hump at 20mph will damage your suspension – you will have to slow about 10mph), or stopping for other cars to pass at narrowed roads and re-accelerating to get moving. This will mean the engine will work much harder than it would have to maintaining a smooth constant 30mph in a high gear where there usually wouldn’t be “traffic calming” in place. (30mph zones rarely have traffic calming measures)

    Thus in the real world to enforce a 20mph zone using the commonly used methods above would actually produce higher emissions.

    The way to reduce emissions would rather be to educate drivers to drop down a gear, and anticipate traffic flow and consciously drive more economically to reduce emissions.

    To give them some props they do admit that a reduction in the speed limit “would have the opposite effect on petrol cars, increasing CO2 emissions from these by around 2%.”

    But isn’t the government heralding the switch from Diesel to Petrol? Yet again the government making rash moves before fully knowing what they are doing.

    Often these blanket statements by campaign groups practically don’t work in the real world but are then immediately accepted and enforced by the government, to the frustration of the typical British motorist, who constantly have to adjust usually at a cost to conform to new legislation.

    Rant over! ha

    • Dani El
      Dani El says:

      That might be because the so called campaigners fail to see the big picture and concentrate on what is an easy fix that doesn’t require too much thinking (that’s hard for some after all)
      Why not start taxing diesel cars properly and make incentives to switch to electric, hybrid or clean petrol/lpg powered cars?


Leave a Reply

Want to join the discussion?
Feel free to contribute!

Leave a Reply

Your email address will not be published. Required fields are marked *