I think the more common expression is "stuck in traffic." The suggestion is that you are in a vehicle, moving very slowly in either very heavy traffic or, if "stuck in a traffic jam" then perhaps you are not moving at all.
I agree. This is a phrase mostly used in America to express the excess of time over the normal time, used for moving in big cities, by a motorist due to traffic jam. On account of sit in traffic, there is considerable loss of fuel as well as productivity. Here is a report on the loss expressed in monetary terms, collected from the web.
The average motorist spent 93 hours sitting in peak-hour traffic in Los Angeles compared with, 67 hours in Washington and 50 hours in New York City.
The annual financial cost of traffic congestion as measured in wasted fuel and lost productivity is estimated at more than $63-billion, compared to $14-billion two decades ago.