High precision GPS on-board
Most automotive GPS systems just need to place a car on the road, not necessarily in a specific lane. Cadillac's Super Cruise needs to know not only what road you are on, but which lane you are in on that road. While typical GPS systems are accurate to within 49.21 feet, Cadillacs equipped with Super Cruise have Trimble high-precision GPS receivers that can place a car within 6.56 feet, or about the width of the average lane on a highway.
General Motors holds the authority
Given this is a connected system, General Motors still holds over-arching authority over how, where, and when it is used. It has the ability to block out certain parts of the map (i.e. long-term construction zones, known issue areas, etc.) making it so Super Cruise either won't work in these areas or will hand back control of the car to the driver upon approaching said areas.
If you feel the need to speed, Cadillac's Super Cruise system isn't going to stop you. The system operates at up to 85 mph, though, if you manually control the throttle and brake, the system will continue to handle steering duties until you hit 92 mph, and then it'll hand full control back to you. Yes, that means if you want to set it to drive at 85 mph in a 55 mph zone, it will. Then again, Cadillac's not going to pay your speeding ticket, so you'll be on your own there, Bucko.
2018 Cadillac CT6Enlarge Photo
Tunnels aren't a problem
For those wondering whether driving through a tunnel is an issue, it's not. The system can continue operating while traveling through a tunnel up to 0.62 miles long. Super Cruise uses a combination of on-board data and Dead Reckoning (a process by which one's position can be calculated—usually used on the sea or in the air—by using the previously known position, course, and speed, then compensating for elapsed time).