Consumer gps doesn't always know which road one is on and could trivially decide to use an adjacent street with a wildly different speed. It's also sometimes u

Consumer gps doesn't always know which road one is on and could trivially decide to use an adjacent street with a wildly different speed. It's also sometimes unclear about where transitions happen. It also never accounts for actual speed of travel diverging substantially because citizens including Police have collectively decided that a different speed is more reasonable. Basically such a system would probably be completely useless enough to ruin all cruise control.