Speed vs velocity formula
Q. A track athlete runs the 1600 meter race (4 laps around the track) in 6 minutes. What is their average velocity? answer choices. 267 m/min. 400 m/min. 0 m/min. 4.45 m/sec.$\begingroup$ @JasonR with the wikipedia model (assuming zero initial covariance between position and speed), the speed estimate is always it's initial value (as you say, a "constant-velocity" model). However, the variance of the speed grows monotonically via the process noise, and there are no measurements which can reduce it.