The cars go forwards; the jam goes backwards, at roughly 15 to 20 km/h — near enough the same whatever the speed limit is.
Queued cars pack about one vehicle length plus a gap apart (~7 m), and each pulls away about one headway (~1.5 s) after the one in front. The back of the queue therefore eats backwards at 7 ÷ 1.5 ≈ 4.7 m/s ≈ 17 km/h. Nothing in that depends on how fast the free-flowing traffic is moving.
It scales with what is IN the queue. A jam full of lorries packs far longer, so its wave runs backwards faster; so does a queue of tailgaters with a short start-up headway, which is the opposite of the intuition that impatience clears a jam. And it only describes the back of a jam that already exists — it says nothing about whether one will form, which is a stability question, not a kinematic one.
Estimate with the rule, then check it against the calculator that models it properly.
Open Traffic Jam Simulator →The cars go forwards; the jam goes backwards, at roughly 15 to 20 km/h — near enough the same whatever the speed limit is. Queued cars pack about one vehicle length plus a gap apart (~7 m), and each pulls away about one headway (~1.5 s) after the one in front.