Eli5: why does traffic form on the other side of the road/highway despite there being no obstruction

Question

I noticed that whenever there’s an accident on the highway, traffic forms on the opposite side despite there being no physical obstruction. And when you pass the accident, people suddenly decide that they’re not in traffic despite there being no different a few feet behind.

In: Other

Answer 1

“Rubber necking”. People slow down to try and glance at the accident….this has an accordion effect