A complete waste of time to slow down when there's no one around. In one town in the states I saw a school zone sign that read underneath "when children present." Brillant!
Remember England tried to lower the speed limit ,but it did not work .
Look at Edmonton Sun April 26 2019 page 7 The myth of speed reduction.
Another waste of taxpayers money you can't get people to slow down posting a speed sign and the cost to replace those signs is unnecessary since most speeders already exceed the the well known unposted limit of 50km so they will have no compunction to exceed 40km. Signs without enforcement is like trying to swim and not get wet.