Even more troubling is the speedometer’s dumbness. The device gives a simple reading that lacks context. It tells speed, but it doesn’t convey other useful information. How does the car’s speed compare to the posted limit? How much time is saved by driving faster, and how does it compare to the added fatality risk of a crash (which rises exponentially at higher speeds)?
As Tom hints, absolute speed is just one input that goes into a driver's calculus of how to drive, and it's often less important than other inputs (such as the current speed limit, current and upcoming traffic conditions, current road conditions, or the total trip time based on the current speed). In other words, drivers need more information besides just raw speed to perform the task of driving. And since today's technology allows for the collection of such information, it seems like a good idea for cars to start providing that information. Of course, it's important to avoid information overload, but clearly drivers aren't currently overloaded with information if they can find the time (and cognitive capacity) to talk/text on their phones, put on makeup, eat, read, etc. while driving.
As Johnny 5 would say, more input!
Image from here.