Name for gap in a line caused by everyone stopping, and then having the front of the line start moving again?

You know when cars stop in a line and then the first car starts moving, causing the rest of the cars to have a big gap at the end? What is that phenomenon called?

A similar example is a line of kids at school. They all stop in a hall, everyone catches up, and then when the front takes off walking the last one or two students in line have a huge gap in between them and the rest of the line, which has moved on.

I studied queue/line flow for my own purposes some time ago.

The phenomenon that causes the gap is called the stop-go wave.

A compressive wave travels back down the line/queue from the first car to stop. This causes the vehicles/people to have a relatively small distance between them. The first car then moves and a decompression wave follows in which the gap between the now moving traffic/people expands.

It is unusual for the gap between a car/person that starts to move and the one behind to vary by much, (people's reaction time is much the same) and so when the queue/line moves, there is not much difference in distance between any two parts of the moving line/queue - thus there is not really, a "huge gap at the end".

If there is a "huge gap at the end" it usually means that the compression wave died down before the end was reached.

There is no word for the gap itself other than "a gap" or "interval" depending on the context.

There are useful technical terms in http://onlinepubs.trb.org/onlinepubs/circulars/ec197.pdf Celebrating 50 Years of Traffic Flow Theory

With regard to a line of people or cars, a noun that could apply to this phenomenon is slack and how line participants react to it and how it seems to accumulate.

slack (n)., take/make up the slack

Cessation in movement or flow

A part of something that hangs loose without strain

A portion (as of labor or resources) that is required but lacking m-w

That part of a rope, sail, etc., which is not fully strained, or which hangs loose; a loose part or end. Also figurative, esp. in to take up the slack, to use up a surplus or make up a deficiency, thereby maintaining or returning to a stable condition; to hold on the slack, to skulk; to be lazy (1864 Slang Dict.). OED

It's like a line of traffic starting up, or the tide turning; first it has to take up the slack, so in the beginning it will seem like nothing is happening. Terry Bisson; In the Upper Room and Other Likely Stories

The two cars in front of me crept forward to take up the slack. R. T. Anthony; Mountain of my Dreams: The Middle Years

At every gap in the traffic, I shot out to my maximum seventy, and by the time he made up the slack, there was more traffic. He was trapped. Robert Morton; Just a Little Watermelon Talk

Danny let the car idle for a moment, then began easing it forward, taking up the slack between him and his new prey. J. Gelb and M. Garret; Hotter Blood

Driving habits in response to high traffic volumes stretch the slack out of the system. Drivers ride on the edge, saving a few seconds or minutes a day, but substantially increasing the risk of much longer delay on some days. T. Moore et al.; The Transportation/land Use Connection

Found thanks to Greybeard's stop-and-go wave:

A number of factors determine how it is distributed between vehicles and how much slack is available as free space. A. M. Okun; "The Game Plan of Stop and Go"

Note that slack was already in use with reference to the cars making up a train:

The question of how to handle the slack between the cars has been given special prominence by a paper by ... Railway Locomotive and Cars, Vol. 32 (1898)