"I am caught/stuck in a traffic jam." You could say "the traffic jam" if there is only one and everyone knows which one it is. Living near Los Angeles, I find it hard to imagine a place where there is only one traffic jam.
👁 Wink ;)
If there were some context, "the traffic jam" might work. "There has been a huge accident involving two big rigs, and I'm caught in the traffic jam behind it."
