From the picture, we can find the distance and time:
The "rate of travel" we've been talking about is an example of an average rate of change, abbreviated "average r.o.c.". When we talk about "rate of travel," we're talking about the average rate of change of distance traveled with respect to time. Similarly, if x is any independent variable (not necessarily time) and y is a dependent variable (not necessarily miles) related to x, we can find the average r.o.c. of y with respect to x on a given interval.