Yes, the constant of proportionality is 3.
In a linear relationship, the accumulated data forms a straight line on a graph. Looking at the given data, we see that for every two elapsed hours, the distance increases by 6 miles. This pattern creates a constant ratio of 6 miles per 2 hours (6:2), and this reduces to 3 miles per hour (3:1). Therefore, there is a linear relationship with the constant of proportionality being 3.