The head of public safety notices that the population average driving speed at a particular intersection averages 35 mph with a standard deviation of 7.5 mph. After a school speed limit sign of 20 mph is placed at the intersection, the first 40 cars travel past at an average speed of 30 mph. Using the .05 significance level, was there a significant change in driving speed?

1. Use the four steps of hypothesis testing. (12 pts.)
2. Sketch the distributions involved. (1 pts.)
3. Figure the effect size (Cohen's d). (3 pts.)