“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.
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a reliable (automobile)
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:
She is highly motivated, reliable, and pleasant to work with.
(an automobile) on the road
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.