[thayne]

As mentioned in another comment there is the WARN act[1] at the federal level. And many states have additional regulations. In practice, employers would often prefer the people laid off just stop working immediately than continuing coming to work knowing they are losing their jobs soon. And I think employers can offer employees a severance agreement where the employee waives their right to the 90 day notice in exchange for some other compensation, such as a lump sum payment.

However, there are exceptions. In particular if the company is small enough, or the layoff is below a certain percentage of employees, it doesn't take effect.

[1]: https://en.m.wikipedia.org/wiki/Worker_Adjustment_and_Retrai...