Crossing Dash

Guide an energy courier across a decommissioned transit grid: freight shuttles that will run you down, open charge conduits you can only cross by riding a carrier plate, and banks to catch your breath on. A hop takes 190 milliseconds and cannot be taken back — and every band is searched and proved crossable before you ever see it.

Powering the courier…

A hop you can take back is not a decision

The usual form of this genre teleports you between cells. A step costs nothing, so a player who is quick enough is never actually wrong, and the only skill left is reflex. Here a hop takes 190 milliseconds and the courier is committed for all of them: it cannot be steered, cancelled or hurried, and a second hop is refused rather than queued.

While it is in the air the courier is safe — drones pass underneath and a stream cannot earth something it is not touching. The danger is the landing. So the cost of a hop is the commitment rather than the flight, which is both fairer and far easier to read: nobody can see why they died halfway between two lanes.

Nothing is generated that cannot be crossed

A generator that rolls plausible numbers and hopes ships the defect this genre is famous for, and it is invisible: the lanes draw, the hazards sweep, the courier moves, and the player dies every time without ever being able to say what they did wrong.

So every band is searched. A breadth-first sweep over lane, position and time — 63 milliseconds a tick, 48 columns — asks whether a courier could reach the bank above within its charge, given exactly where every drone and plate will be. Because where the hazards are is a pure function of the lane and the clock, the search never has to simulate anything, which is what makes proving a band cheap enough to do while you play.

The search rounds against itself: the courier is treated as one column wider than it is and every plate one column narrower, so a crossing it certifies is one the real courier definitely makes. A solver that rounded in its own favour would occasionally put a proof next to an impossible band, which is worse than having no solver at all.

From wherever you are, not merely from somewhere

The first version of the check asked whether some starting column worked. The answer was yes four hundred times out of four hundred — with 48 columns to choose from, one of them always lines up and the courier can walk straight up without waiting once. But you do not choose where the previous band left you. A band crossable from one lucky column and impossible from the other ten kills people for a position they did not pick, which is the exact unfairness the check exists to prevent, so it now has to pass from 8 columns spread across the grid.

A gap holds three things, not one

A landing, a look, and a departure — 0.63 seconds. The first draft asked only for a hop plus a reaction, on the reasoning that the arriving hop is free because the courier is airborne during it. That is true, and it misses the other half: having landed and looked, you still have to wait for the lane above to open, and that waiting happens standing in the gap. The difference was not academic. At the shorter figure the lanes were each individually legal and collectively merciless, and a competent test bot died every two and a half seconds.

Local play

Everything runs in this browser. There is no sign-in, no leaderboard and no account: your settings, your bests and which Daily grids you have crossed live in local storage under this route’s own keys, and clearing this game’s data leaves every other Zerdly tool alone. You get 3 couriers in most modes, every 5th band is a checkpoint you return to rather than starting over, and each band in an unbroken run adds 40 to the next up to 15 deep. The grid is 11 columns wide.

Questions

Why does a hop take time instead of being instant?: Because an instant step is not a decision. If moving costs nothing, a player who is quick enough is never actually wrong, and the only skill in the route is reflex. A hop here takes 190 milliseconds and the courier is committed for all of them — it cannot be steered, cancelled or hurried, and a second hop is refused rather than queued. That changes the question from "is the gap there now" to "will the gap still be there when I land", which is a better question and the one everything else is built around.
Am I safe while I am in the air?: Yes. Drones pass underneath and a charge stream cannot earth a courier that is not touching it — the danger is the landing. That makes the cost of a hop the commitment itself rather than the flight, which is both fairer and much easier to read: nobody can see why they died halfway between two lanes. It is not a loophole either, because every hop ends in a landing and every landing is checked, so you are tested every 190 milliseconds however you move.
Can a band be impossible to cross?: No, and the generator is asked rather than trusted. Every band is searched before it is used: a breadth-first sweep over lane, position and time that asks whether a courier could get from the bank below to the bank above within its charge, given exactly where every drone and plate will be. A band that fails is thrown away and another is rolled. The check is kept honest by being run against bands that are impossible on purpose — a wall of drones with no gap, a stream whose plates are too small to stand on — which it correctly rejects.
Crossable from where, though?: From wherever you actually are, which is a much stronger requirement than it sounds. The first version of the check asked whether SOME starting column worked, and the answer was yes four hundred times out of four hundred — with 48 columns to choose from, one of them always lines up. But you do not choose where the last band left you. A band crossable from one lucky column and impossible from the other ten kills people for a position they did not pick, so the check now has to pass from 8 columns spread across the grid.
What stops me waiting for a perfect gap every time?: Charge. The courier carries 9 seconds of it, draining whenever it is not on solid ground and refilling the moment it reaches the next bank — so a crossing is a budget rather than an open-ended wait. Reaching a bank with charge to spare is also worth points, so hesitating costs twice. And the fairness search is given exactly those 9 seconds, because a proof that a band is crossable in more time than the courier has would be a proof about a different game.
How fast can a hazard travel?: No faster than 1.45 grid-widths a second, and that number is derived rather than chosen. Hazards are drawn 14% of a grid-width beyond each edge, so the worst case is one entering at the edge heading for a courier in the middle: it has to stay visible for 250 milliseconds of reaction plus the 190 milliseconds of the hop that gets you clear. The difficulty ramp climbs from 0.32 toward that ceiling and is clamped at both ends.
What makes a lane survivable on its own?: A gap has to hold three things, not one: a landing, a look, and a departure — 0.63 seconds in total. The first draft asked only for a hop plus a reaction, reasoning that the arriving hop is free because you are airborne during it. True, and it misses the other half: having landed and looked, you still have to WAIT for the lane above to open, and that waiting happens standing in the gap. The difference was not academic — at the shorter figure a competent test bot died every two and a half seconds.
What do the carrier plates do?: A stream is an open charge conduit: land on a plate or the courier is earthed. Standing on one is support in the honest sense — your centre has to be over it, not merely touching it — and the plate then carries you sideways whether you like it or not. If it carries you off the edge of the grid it takes you with it, which is a real way to lose and is modelled in the fairness search as well as in the game. Some bands can only be crossed by riding a plate across and hopping off the far side.
Do the modes actually differ?: Endless, Stage Run, Daily Crossing, One-Life. Endless has no finish and keeps widening; Stage Run is 24 bands with checkpoints and an end you can reach; the Daily is 16 bands seeded by the UTC date, so it is the same grid for everybody that day; and One-Life gives you a single courier and no checkpoints.
Is the grid the same every time?: Only the Daily. Everything else is seeded fresh, and bands are generated ahead of you as you climb — between 2 and 5 hazard lanes each, widening with the difficulty. The same seed always produces the same grid, which is what makes a Daily a Daily and what lets the fairness proof mean anything.
Can a hazard pass through me?: No. The simulation runs at a fixed 180 steps a second whatever the display does, and where every hazard is at any instant is a pure function of the lane and the clock rather than a list of objects being moved about. That is also why the picture cannot disagree with the collider: what is drawn and what you collide with are the same function, asked the same question.
Can I play without a mouse?: Yes. Arrow keys or WASD hop, and a held key does not repeat — one press is one hop. On a touch screen you can swipe the grid in the direction you want to go, resolved when you lift your finger so a hesitant diagonal is never read as two hops, or turn swiping off in settings and use the four pads under the grid. Forward is the wide pad, because nearly every hop is forward.
What is stored about me?: Nothing that leaves this browser. Your settings, your best for each mode and which Daily grids you have crossed are kept in local storage under this route's own keys, and clearing this game's data leaves every other Zerdly tool alone. No run in progress is ever saved — the seed is stored and the grid is rebuilt from it, so a courier can never resume onto a band nobody proved. There is no sign-in and no leaderboard, because there is no server to hold one.

Related interactives

Space Defender — The other route where a committed move is the whole decision.
Endless Highway Racer — Traffic again, from inside it rather than across it.
Traffic Escape — The same jam, standing still, solved with your head instead.
Reflex Grid — The route that measures the quarter-second this one designs around.