toltec wrote:
... you do not necessarily need a speedometer to drive safely. Even at higher speeds where it is much harder to judge closing speed to distant objects you can get sufficient information from the engine note and knowledge of what gear you are in.
A speedo would be useful for calibrating your perceptions in a new car so having one is still handy but you should not need to look at it unless you need to know your speed to some precision, as when passing a camera for instance.

Near as I can tell, speedometers are most useful when significant changes in speed occur very quickly - on / off ramps, mostly.
For those of you that saw that episode of Top Gear ...
After having achieved 253 MpH in
the Bugatti Veyron 16.4, Jeremy Clarkson slowed down to about 70 MpH, and then proceeded to open the door. His vestibular sense of motion thought he was standing still, and his visual sense of motion was momentarily stunned out of focus by having recently done 253 MpH. (I'm sure when he got out, he thought he was moving backwards.)
Had he looked at the speedometer, his perceptions would have been checked, and he would not have opened the door prematurely.
When we drive, we regularly exceed our biologically intended velocity. Speedometers acknowledge that novice perceptions need be calibrated (and not just to the next car).