The LabTRAFFIC WAVE SIMULATOR SCIENCE · RING ROAD

A JAM WITHOUT A ROADBLOCK

One brake tap.
A jam out of nowhere.

Twenty-two identical drivers. One circular road. Nobody crashes and no lane closes, yet a jam appears, and it drives backwards.

START WITH A QUESTIONPredict · press play · watch the lines
MODEL TIME0.0 s
Average speed0km/h across all cars
Slowest car0km/h
Flow0cars per hour past a point
Stopped0cars under 2 km/h
Jam wavenoneno queue to track yet
FROM ABOVE · 240 M, ONE LANE

The ring road

Tap any car to make it brake.

CAR SPEED01530 km/hbrake lights
EVERY CAR, EVERY MOMENT

Space–time diagram

Each line is one car moving up the road over time. Red and orange bands are queues; watch them slide down the chart: the jam moves backwards while every car moves forwards.

WHAT’S HAPPENING

Calm for now.

Press play to start the clock.

THE BALANCE POINT

If nobody ever braked.

Steady speed
–
Bumper gap
–
Flow
–

The speed at which every car is exactly content with its gap. The question is whether that balance survives a tap.

PREDICT FIRST

TRY THIS

Three small challenges.

    UNDER THE BONNETHow the model works

    Every car runs the same rule, the Intelligent Driver Model. Each moment, a driver looks only at the car directly ahead: its gap and how fast it is closing. They accelerate towards their desired speed, but brake when the gap gets smaller than they want.

    The desired gap is a 2 m jam distance plus speed × time gap, plus an extra margin when you are closing on the car ahead. That extra margin is the key: a driver who sees the car in front slow down brakes slightly harder than needed. Pass that overreaction down a line of cars and a gentle tap grows into a full stop.

    The road is a 240 m loop, much like the 2008 experiment in Nagoya, Japan, where 22 real drivers circling a 230 m track produced a jam with no cause. On this loop, the jam has nowhere to escape, so you can watch it circle.

    KNOW THE LIMITSWhat this does not show
    • Identical cars and drivers on one lane: no lane changes, trucks, junctions or traffic lights.
    • No reaction delay: drivers respond instantly, so real waves can be worse.
    • Cars update together in steps of at most 0.05 s, with smaller steps if a gap gets unsafe. A safety check stops the run rather than allow an overlap.
    • The brake tap is a deliberate intervention, not a model of distraction.
    • Flow is density × average speed. Jam-wave speed is estimated by tracking the slowest point of the queue.
    • It explains a mechanism. It does not predict a real road.
    GO DEEPERSources & notes

    Model parameters here: jam distance 2 m, comfortable braking 1.8 m/s², car length 4.5 m, acceleration exponent 4.