A road on our ground is a curve made wide in the one direction across it. On a four-dimensional ground there are two directions across a curve, so this track is a curve made wide in both: a tube of tarmac marked out on the flat floor of a closed basin you can't climb out of.
You see the 3D slice you stand in. The track's bends turn partly in your slice and partly into w, where you can't see them, and the plane of each bend keeps turning as you go round. That turning is torsion, and no road on our ground has it. It's what a bicycle here has to steer by. A bicycle rides it, balanced and steered by its own dynamics: walk beside it, or ride it: the stick is the handlebars.
The map is the whole ground, x, z and w, with w up: the track drawn as the tube it is, its true thickness, with round cross-sections every few metres, coloured by w. The gold dot is you, the gold tick where you face, the dashed blue the direction you can't see. Drag it to turn it.
Walking: WASD walk (shift runs) · mouse turns and looks (click to grab it) · Z/C or the wheel twist · Q/E look into w · X square up · T face the track. R switches between walking and riding. Riding: WASD or the arrows push the handlebars (A/D across your slice, W ana, S kata; let go and it's hands off), V the camera, and under ⚙: who holds the bars (H: easy, where the stick is the lean and the bike takes it at once; hands on, where it pushes the bars; assisted; autopilot), the steering (M), tilt, and the limits (off: it never falls and leans however far you push, flagged ⚠ past 46°; real: it falls). On a phone the stick walks when you walk and is the handlebars when you ride; drag anywhere else to look. how it works · kata · IRIS⁴, whose engine this is