Final answer:
The service road is used for both entering and exiting a highway, serving as a transition area before merging onto or off of the main highway lanes.
Step-by-step explanation:
The part of the highway that is used for both entering and exiting a highway is the service road. It allows vehicles to slow down from highway speeds before exiting or to accelerate to highway speeds before merging onto the main lanes of the highway. On-ramps are specifically designed for entering, and off-ramps are for exiting the highway. Express lanes, on the other hand, are meant to facilitate faster through traffic and typically do not provide direct access for entering or exiting.