I assume you mean traffic on the roads. You might say heavy, bumper-to-bumper, snarled, or congested. On the other hand, traffic might be light or flowing smoothly.

a busy traffic?

You might say: "I'm going to be late as I'm caught in a terrible traffic-jam because it's rush-hour here." You could also talk about: traffic circulation, traffic congestion, traffic lights, gridlock, traffic density. Alternatively, what is traffic? Lorries, cars, buses, commuters, vans, pedestrians, vehicles, transportation