Learn Spanish (Spain): El tráfico en la ciudad es muy peligroso. - The traffic in the city is very dangerous.

We use muy (very) because it comes right before an adjective (peligroso). You use muy to increase the intensity of adjectives and adverbs. You would only use mucho (a lot / much / many) if you were talking about a noun, for example if you wanted to say there is a lot of traffic (mucho tráfico).

Why is it peligroso and not peligrosa, since la ciudad is feminine?

Adjectives must match the gender of the noun they are actually describing. Even though la ciudad (the city) is the closest noun in the sentence, the word peligroso (dangerous) is describing el tráfico (the traffic). Since tráfico is masculine, the adjective must be masculine too.

Wait, why is it la ciudad? Doesn't it end in a consonant?

Yes! While most feminine nouns end in -a, Spanish has a few extremely common rules for other endings. Nouns that end in -dad (like la ciudad or la universidad) are almost always feminine.

Why do we use es here instead of está? Isn't traffic a temporary thing?