If you reduce cars to a length of just 1, you get Unit Rush Hour, which turns out to be much harder [1]. Thew hardest 6x6 instance takes 732 moves to solve!
Unit Rush Hour can also be viewed as a maze with a certain orientation rule.
On my orimaze page you can try playing the hardest 4x4 and 5x5 instances in your browser.
[1] https://arxiv.org/abs/cs/0502068
[2] https://tromp.github.io/orimaze.html
