“They're one of the most reliable cars on the road.”
Your friend just bought a new car. The model that she bought is a good one which rarely breaks or has problems. You comment on her choice.
They're one of the most reliable cars on the road.
When a car or truck is well-made, you can trust that it will not break down. You can call a vehicle which doesn't break down easily "reliable".
You can also call a person reliable:
Use the phrase "on the road" when talking about cars and other automobiles.
This phrase is useful for contrasting a certain automobile with all of the other ones:
It's one of the fastest cars on the road.
This means that it's one of the fastest cars that people commonly drive.