The demo drives itself

/race is a presentation, not a game — so it starts on autopilot and drives itself. The controller is deliberately small. This post is what each part is doing and what a real one adds.

Steering is an error signal

const desired = Math.atan2(aim.x - pos.x, aim.y - pos.y);
let err = desired - heading;                       // normalize to [-π, π]
steer = clamp(err * 2.5, -1, 1);                   // k_p · e

That's the whole steering law: a P-controller. err is the angle between the car's nose and the aim point; multiply by a gain, clamp to full lock. The while loops handle wraparound so a target 10° right doesn't spin the car 350° left.

What it doesn't have is the D term — a derivative to damp the approach — which is why it can wobble slightly into tight corners. The fix is one more line and a tuning session; for a demo it reads as characterful rather than broken, and it's honest about being P not PD.

The aim point is the racing line

The car doesn't chase gates — it chases the road. The aim is a centerline sample a look-ahead distance away, and the distance scales with speed:

lookI = nearest.i + (|speed| · 0.55) / segLen;
aim   = track.pts[lookI].p;

Slow → aim nearby, tight control. Fast → aim far ahead, which cuts a smooth line through corners the way a human driver looks through a turn rather than at the cone in front of the bumper.

Braking for the corner you can see

Speed control is a min-scan over the look-ahead window:

vTarget = min over next window of sqrt(GRIP · radius)

Find the tightest upcoming radius, compute its corner speed with the same v_max formula the crash rule enforces, and brake to meet it — full throttle when below target, brake when above, cruise at the boundary. One number (vTarget) is the entire traction policy.

The result is genuinely instructive to watch: the car visibly lifts before the hairpin, brakes harder the faster it arrives, and opens up on the straights. The autopilot respects the same physics sheet the site prints — which is the point. The controller isn't a gimmick layered on the product; controller · core components is the product.