Speeds are enforced with automatic enforcement (video-based time over distance cameras in the more recent schemes). The use of the hard shoulder for through traffic at times of high congestion is a common feature. The aim of ISA is to discourage or even prevent speeding, by informing drivers about the speed limit for a road, warning them about excess speed and in the most sophisticated systems preventing speeding by use of an electronic speed limiter. The fundamental distinction in ISA is between advisory (warning) systems, which typically beep at the driver when speeding over a certain threshold is detected, and intervening systems, which typically limit speed at, or just over, the speed limit.