Why do we say "traffic jam"?

A thought occurred to me during a tedious journey yesterday, when travelling why do we use the word jam when describing being

…caught in a traffic jam?

It is just a queue, in this case, it happens to be composed of traffic but if I was in a shop waiting in a queue of people I wouldn’t say I was in a people jam or queue jam... why not?

The sense of jam in traffic jam meaning traffic congestion developed out of the verb connotation of obstructing, blocking or become immovable. The expression dates back to the early 20th century.

As The Word Detective explains:

“Jam” first appeared in the early 18th century as a transitive verb meaning “to press or squeeze something” or “to wedge or immobilize something in an opening”. The Oxford English Dictionary (OED) labels it “apparently onomatopoeic”.

• (“The Ship … stuck fast, jaum’d in between two Rocks,” Defoe, Robinson Crusoe, 1719).

“Jam” as a verb went on to mean “to block or obstruct” (eventually producing the “jamming” that can block radio signals) and, as an intransitive verb, meaning “to become immovable or unworkable by wedging or sticking” as a gun may “jam.”

• As a noun, “jam” developed a variety of meanings, most of them involving either the act of “jamming” or the result of “jamming,” as in a “traffic jam” or, in a figurative sense, “jam” meaning a difficult situation (“I’m in a jam. But I’m not going to the cleaners… Half of this money is mine,” Raymond Chandler, 1950).