=+ In one investigation, the authors selected six sites along Virginia interstate highways where service signs are posted. For each site, crash data was obtained for a three-year period before distance information was added to the service signs and for a one-year period afterward. The number of crashes per year before and after the sign changes were made are given here:

Before: 15 26 66 115 62 64 After: 16 24 42 80 78 73