Surely the worse offenders pollution wise are lorries/buses/vans etc
As ahwiles has implied, I think actually modern diesels are probably the worst offenders for NO2, which the consultation document says is the primary pollutant of concern for that stretch of road.
(The NAEI figures for 2011/2012 imply EURO 4/5 diesel cars contribute around 0.3 g/km of motorway travel versus around 0.4 g/km for HGVs, but there are probably 4-5 times as many diesel cars travelling on that stretch of the M1 on average; so the NO2 contribution is much larger from diesel cars.)
Reducing the average speed from 70mph to 60mph would reduce NO2 emissions by over 20% for/from the 2012 average UK diesel car fleet. For petrol it’s more like a 10% reduction in NOx/NO2. (Citing the UK EFT v5.1)