Let \( t \) be the time in hours since they started the trip. The piecewise function \( D(t) \), representing the distance traveled in miles, can be defined as:
\[ D(t) = \begin{cases} 55t & \text{for } 0 \leq t < 1 \ 55 + 70(t - 1) & \text{for } 1 \leq t \leq 3 \ \end{cases} \]
This function accounts for the first hour of travel at 55 mph and the following hours at 70 mph.