Schematic of the (1+1)D "traveler-kinematic"P. Fraundorf (2012) "A traveler-centered intro to kinematics", arxiv:1206.2877 . (centered around "τ", dx/d"τ" & α) for describing motion regardless of speed. The key variables here are traveler or proper-time "τ", proper-velocity w ≡ dx/d"τ" and proper-acceleration α. The traveler-kinematic approach works also in (3+1)D flat space-time, and should in principle work with other metric equations as well. The "map-kinematic" of Galileo & Newton (centered around t, dx/dt & d2x/dt2) is of course the familiar low-speed approximation.