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!<p>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.<p>[1] <a href="https://arxiv.org/abs/cs/0502068" rel="nofollow noreferrer">https://arxiv.org/abs/cs/0502068</a><p>[2] <a href="https://tromp.github.io/orimaze.html" rel="nofollow noreferrer">https://tromp.github.io/orimaze.html</a>