It seems obvious that a 20mph limit will be more safe in most instances.

There’s the issue with older cars having a gearing not really suited to staying at that speed (meaning revving higher and chucking out more pollution/being louder), but that’s less of an issue with new cars.

There’s also the issue of a car in 2nd gear at 20mph being ready to take off far more easily if you were to accidentally press the accelerator instead of the brake – something which often happens in traffic incidents. But again, the gearing of more recent cars is typically fine with sitting at 20mph in third gear. Cars seem to be designed with 20mph in mind now.

And honestly, in most circumstances it won’t make a serious difference to travel time going 20 or 30. Either way you’re likely going to be stopping multiple times or stuck behind people.

Worth noting though, that the very people who collected this data say that the data can’t really be used for direct comparison and that we need at least a couple more years to see what the effect of the change is.