I'm writing a short paragraph about my city and I want to mention the traffic. I remember there being a word to describe a situation that's even worse than a traffic jam, where vehicles can't move at all and are completely stuck, but I can't remember what it is.
You may be thinking of gridlock
defined by Dictionary.com as:
1. the stoppage of free vehicular movement in an urban area because key intersections are blocked by traffic.
2. the blocking of an intersection by vehicular traffic entering the intersection but unable to pass through it.
Here’s a great picture from timlane.org: