Learn French: La circulation, elle est infernale aujourd'hui !

This is a common spoken structure called left dislocation. Instead of just saying La circulation est infernale, the speaker pulls the topic (La circulation) to the front of the sentence to emphasize it, and then refers back to it with the matching subject pronoun (elle). It gives the feeling of saying "As for the traffic, it is awful!"

Is it grammatically correct to double the subject like this in French?

In standard, formal written French, you would avoid this and simply write La circulation est infernale aujourd'hui. However, in everyday spoken French, doubling the subject to show emphasis is extremely natural and common. You will hear French speakers use this structure constantly in daily life.

Why use circulation for traffic instead of trafic?

For everyday car traffic on the roads, the correct word is la circulation. The word le trafic is usually reserved for large-scale transit networks (like air or rail traffic), commercial trade, or illegal smuggling.

Why does infernale end with an e?

Because infernale is an adjective describing La circulation, which is a feminine noun. Even though the noun is pulled to the front of the sentence as a topic, the adjective in the main clause still agrees with it in gender and number.