Have you ever noticed how, on a highway in conditions of heavy traffic, whenever you have to brake for any reason, the braking turns out to be sudden and hard?
The same does not seem to happen on slow and/or less busy roads. Can you explain the difference?
P.S: strictly speaking, this is not a physics problem but in the theory of queues, but the queue tag does not exist.
Try to imagine under which (physical) conditions a smooth section of traffic is brought to a sudden stop rather than a mild slow-down.