The 101 Plays Itself

What you are hearing

A camera on a pole above the 101 watches five lanes of traffic. Software follows every vehicle, and the moment one crosses an invisible line drawn across the road, it plays a note. The lane it is in decides the pitch. How fast it is going decides how long the note holds.

Nothing here is composed, and nothing is pre-recorded — the audio is synthesised in your browser from the tracking data as the video plays, which is why you can change instruments mid-piece and hear the same traffic played a different way.

The score key

Pitch

Which lane. The fast lane sings highest; the slow lane lowest.

Voice

What kind of vehicle. Trucks drop an octave, motorcycles jump two.

Length

How fast it was moving. Speeding cars clip short; slow ones hold and bend.

Volume

How close it passed. Nearer and larger reads louder.

Sharps & flats

Changing lanes. Moving left bends the note up a semitone, right bends it down.

Stereo

Where it sat across the road. The far shoulder is hard left; the near barrier hard right.

How it knows where the lanes are

Nobody drew them. The software watches a few hundred cars, finds the point on the horizon where all their paths converge, and works backwards to the lanes from there — they turn out to be evenly spaced, 98 pixels apart in the corrected view.

Two numbers say whether it worked. Told only that a US freeway lane is twelve feet wide, it solved the camera to 7.0 metres above the roadway — about right for a Caltrans pole — and the traffic to 67 mph, which is what the 101 does at midday when it is behaving. Neither number was given to it.