Mean speed
Slowest car
Density
0 m/s

Presets

Each row is a 10 km ring with the default drivers, started at its settled speed. One car braked once and the traffic was watched for 60 simulated minutes. Sparse traffic absorbs the brake. From about 23 cars/km a 5 s brake grows into a stop-and-go jam, and from 28 cars/km even a 1 s brake does. That tipping point sits at the model's peak flow, about 1,830 cars an hour.

Pick an outcome to load that density and brake duration with the default drivers. Then press Play and click a car. Times are simulated; a jam means the slowest car fell below 5 m/s.

About this simulation

The road
The ring stands in for a long stretch of motorway with no junctions: one lane, identical drivers, no overtaking. Cars are coloured by speed, and anything at 27.8 m/s (100 km/h) or faster is the same green.
Starting speed
Every run starts with the cars evenly spaced at the speed traffic settles to at that density, so nothing happens until a car brakes.
Braking
Clicking a car makes it brake at 2 m/s² for the chosen duration. Clicking a car that is already braking keeps whichever end time is later.
Traffic at the speed limit cannot jam with this driver model
Cruising at 70 mph needs traffic as sparse as about 8.5 cars/km, and at that density disturbances die out: a 10 s brake among 85 cars on 10 km faded away. Jams need busy traffic, at around 45 mph or slower.
Desired speed barely matters in busy traffic
Raising it from 33 to 36 m/s changed little about when jams formed at 30 cars/km and denser. There the gaps between cars set the speed, not what the drivers would like to do.
Sliders
Driver sliders take effect immediately, mid-run. Road sliders restart the run. Brake duration applies to the next click.
Run speed
64× needs about 60 frames a second. On a slower display the simulation tops out lower instead of skipping ahead.
Collisions
Extreme driver settings can make cars collide. The run then stops; adjust the sliders and press Reset.