If you travel at 60 mph instead of 70, you'll spend 16% longer time
covering a given stretch, will the NOx emission rate per mile be reduced
sufficiently to have much overall effect on the total emitted over a 4.5
mile stretch?

Fig 5 of this seems to show under a 10% factor on the NOx between the
relevant speeds

https://escholarship.org/content/qt01x9w3kf/qt01x9w3kf_noSplash_cbac2ea8aba905cf6ef7d277318129 33.pdf