Traffic Jam — the free sliding block puzzle where you drive the red car out

A six-by-six car park, jammed solid. Every vehicle is stuck on its own axis: the ones lying across the board slide left and right, the ones standing up slide forwards and back, and none of them can turn, lift or pass through anything. Somewhere in the third row is a red car, and directly opposite it is the only gap in the wall. Shunt the traffic out of its way and drive it out.

There are ten levels here and they get properly hard. Each one carries a move budget, so a run has an ending: waste enough slides and you finish with the levels you already cleared as your score. Every board was solved by exhaustive search before it shipped, so the shortest solution is a known number and the budget is generous against it — two and a half times the minimum, rounded up. Free, no download, and it plays with one thumb.

How to play

• Drag a vehicle along its lane. It stops against the next car or the wall, and snaps to whole squares.
• Get the red car to the gap in the right-hand wall to clear the level.
• Prefer tapping? Tap a vehicle to select it, then tap an empty square in its lane and it drives there. Arrow keys move the selected vehicle too.
• One slide costs one move, however many squares it covers. Moves left are shown above the board.
• Undo is unlimited — but it puts the board back, never the move. A move you undo is still spent.
• Restart level resets the cars to their starting positions and does not refund the moves you have already spent.
• Clear a level and the next, harder one is dealt. Your score is levels cleared, and running out of moves ends the run.

Work backwards from the exit — the only cars that matter

The instinct on a board of twelve vehicles is to scan the whole thing looking for something that can move. That is the slow way, and it is why the move budget bites. The fast way starts at the exit and walks backwards. Look along the red car's lane, to the right of it, and list what is sitting in those squares. Those vehicles — and only those — are the ones that must move. Across the ten levels here that list is usually two vehicles, sometimes only one, and never more than four. Everything else on the grid is scenery, right up until it turns out to be blocking one of the blockers.

Then ask where each blocker can actually go, and the arithmetic is far tighter than the board looks. A blocker is always standing vertically — nothing lying horizontally ever shares the red car's lane, because a vehicle in that lane could only ever slide along it, so it would either sit between the red car and the gap forever or be scenery that can never matter. Each blocker therefore has to clear the third row by moving up or down, and the board is only six rows deep. A three-square truck crossing that lane has exactly one legal destination: the bottom three rows. It cannot clear upwards, because there are only two rows of space above the lane and it needs three. A two-square car has three destinations — the top two rows, or either of the two positions below the lane. One, or three. That is the entire branching factor, per blocker.

Now recurse. What is stopping that truck dropping to the bottom? Usually a single vehicle, and that one usually has one legal destination of its own. Follow the chain and a board that looked like a wall collapses into a handful of forced moves — which is why level four here, nine vehicles wedged into thirty-six squares, comes apart in nine. You are not searching the board; you are resolving a short chain of dependencies, and the only real decision is the order you resolve them in, because two blockers often want the same square.

Three habits follow from that. Do not fidget with the red car. Backing it up is sometimes genuinely part of the answer — on half of these ten levels the shortest solution reverses it at least once, because that is the only way to free a square a truck needs to drop into — but it is never progress on its own, and every idle nudge costs a move you cannot get back. Slide the full distance in one drag, because one slide costs one move whether it covers one square or four, and four-square slides do appear in these solutions; nudging a car two squares and then two more pays twice for the same result. And plan before you touch anything: undo will restore the board but not your budget, so the score really measures how confidently you spotted the chain, not how fast your thumb is.

More games

More puzzles about moving one thing without wrecking another: Sokoban, Block Puzzle, or the 15 Puzzle. Or browse all games.