A classic jam
Sliding-block traffic puzzles have been around for a long time, as wooden trays and as boxed games. The idea is always the same. Vehicles fill a small lot. Each one can only roll forward or back in its own lane. One special car has to get out. On this 6×6 lot, the red car sits in the third row from the top, and the exit is the gap in the right wall at the end of that row. Cars take up two squares and trucks take up three.
Nothing ever turns, so every vehicle is stuck on one line. That makes the puzzle easier to think through than it looks. For each vehicle in the way, you ask one question: where along its own line could it go?
Start from the exit
Don't start by moving whatever looks free. Start at the red car and look right. Every vehicle in the third row between the red car and the exit has to leave that row. Since they all stand upright, each one must slide up or down.
This is where length matters.
- An upright car covers two squares. To clear the third row, it can go up into the top two rows or down into the lower three.
- An upright truck covers three squares. There are only two rows above the exit row, so a truck can never clear it by going up. It must slide down until it sits entirely in the bottom three rows.
Now look at what stops each blocker. If a truck in the exit row must go down, check which vehicles sit in its column below. Each of those has to move sideways out of the column first. This builds a chain: the red car needs the truck to move, the truck needs a car below it to move, and that car needs space in its own row. Follow the chain until you reach a vehicle that can move freely. Then work back up.
Here is a short example. Say a truck stands in the fourth column, covering rows one to three. It blocks the red car. It can't go up, so it must slide down three squares into rows four to six. In row five, a car lying across sits in that column. Before anything else, that car has to slide left or right out of the truck's way. If there is room on one side, that is your first move. Then the truck drops. If the lane is otherwise clear, the red car rolls out.
Making every move count
One move is one slide of one vehicle, however far it goes. Par, the proven fewest moves, follows that rule too. So:
- Slide fully. When a car needs to clear a lane, push it as far as it needs to go in one drag. Two short slides of the same car cost two moves.
- Don't park in a lane you will need. Before you let go, check whether the new spot blocks the chain you just planned.
- Move only what the chain needs. A car that isn't part of any chain doesn't need to move. Extra moves feel productive but take you further from par.
Where players get stuck
The usual trap is going in circles. You move a car to free one lane, which blocks another, then move it back. If the same car keeps going back and forth, stop and redraw the chain from the exit. Look for a vehicle that needs to go the opposite way.
Another trap is assuming a truck can go either way. Check its length before you plan. Undo steps back through every move, and Reset puts the lot back as it was dealt. Hint works out the best next step from where you are, then rings one car and names its direction. Use it to find a lost chain, not to replace one.
Moving up the difficulties
Easy, Medium and Hard are graded by par, the proven minimum for each deal. Easy boards fall to a short chain. Hard boards are more crowded, and you will often move cars just so other cars can move. Your fewest-moves best is kept for each difficulty on this device. Once you can finish Medium near par, try Hard. If you like thinking several pushes ahead, Sokoban uses the same planning with crates that can only be pushed.
