Learn Spanish (Latin America): El tráfico en el centro es muy pesado.

We use muy (very) because it comes right before the adjective pesado (heavy) to describe how heavy the traffic is. You only use mucho (a lot) when talking about nouns, like saying there is a lot of traffic (mucho tráfico).

Does pesado literally mean heavy like a physical weight?

Yes, pesado relates to physical weight, but in Spanish it is also used metaphorically. Just like in English, we say traffic is pesado (heavy). You can also use it to describe a person who is very annoying!

Why is it es muy pesado and not está muy pesado?

Using es (from the verb ser) means we are talking about a permanent or general characteristic. It tells us that traffic downtown is generally always heavy. If you were stuck in traffic right now and it was unusually bad, you would use está (from estar) to say the traffic is heavy at this specific moment.

Does el centro just mean the exact middle of something?

It can mean the middle, but when talking about a city, el centro means downtown. So en el centro translates perfectly to downtown or in the city center.