First answer is simplicity: you have a few prototypical road types and speeds and easy to remember round numbers. E.g. 30 signs are common for inner cities, 50

First answer is simplicity: you have a few prototypical road types and speeds and easy to remember round numbers. E.g. 30 signs are common for inner cities, 50 for outer, but 40 or non-multiple of 10 is extremely rare. That is also reflected in gears for cars, i.e. going at those speed requires very little active input by the driver while going at exactly 42 would need constant manual adjustment. > It seems more sensible to make the the change more slowly and study the results. That was already done locally and other countries exist. So you would have to argue what makes Ireland that unique that further experiments are needed.