5 pm-6 pm is one hour and they measure service in 15-minute blocks so 60/15 = <<60/15=4>>4 15 min blocks
At peak times, they serve 12 cars every 15 minutes so they serve 12*4 = <<12*4=48>>48 cars in 60 minutes
Off peak times, 4pm-5 pm, they serve 8 cars every 15 minutes so they serve 8*4 = <<8*4=32>>32 cars in 60 minutes
From 4pm-5 pm they serve 32 cars and from 5pm-6 pm they serve 48 cars so they serve 32+48 = 80 cars
#### 80