Pick a road or a lesson
Choose the ring road, the on-ramp, the traffic lights or the roundabout, or open a preset such as Phantom traffic jam or Zipper merge. The traffic starts right away.
Free Traffic Simulator
This traffic simulator lets every car drive on its own: it follows the car ahead, reacts a moment late and changes lanes when it pays. Start on a ring road where a phantom traffic jam forms without any accident, then try an on-ramp, traffic lights and a roundabout. Change the traffic, the speed limit, the driving style, the reaction time and the signal timing, stall a car, and read the average speed, flow, density and delay live, with a time-space diagram that shows jams rolling backwards. Save a picture or a video clip.
Open Traffic Simulator →How to use the traffic simulator
Change one thing
Move a slider: more cars, a lower speed limit, bolder drivers, slower reactions or a longer signal cycle. Stall a car to block a lane. Most changes apply while the traffic keeps running.
Read the result
Watch the average speed, flow, density, throughput and mean delay, and the time-space diagram where jams show as red bands. Save a PNG or export a video clip.
What the traffic simulator shows
Jams from nothing
On the ring road there is no accident and no narrowing. At 80 km/h and a 1 second reaction time, from about 35 cars per km per lane small speed changes grow until cars stop, and the stop travels backwards around the ring. With fewer cars the same drivers flow smoothly.
Reaction time decides
Quick reactions has the same number of cars as Phantom traffic jam, but drivers respond within 0.2 seconds. The waves fade instead of growing.
Zip at the end
When ramp drivers use the whole merge lane and take turns at its end, the merge keeps moving. Merging early leaves the lane empty, the ramp queue grows and the mean delay goes up.
Pick a sensible cycle
Every change of signal costs yellow and all-red time. A 30 second cycle loses so much green that queues keep growing, and 180 seconds makes everyone wait through long reds. For the preset traffic, about 60 to 90 seconds works best.
Roundabouts keep cars rolling
At the same moderate traffic a roundabout lets drivers go whenever there is a gap, so the mean delay is lower than at the traffic lights, where every driver who arrives on red waits for green.
One stalled car
Stall a car on a two-lane road: drivers squeeze into one lane, the flow drops and a queue spreads back. On a single lane everybody waits until you click the car again.
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Traffic simulator FAQ
Is this traffic simulator free?
Yes. Every road, preset, setting and export is free, with no account, no sign-up and no watermark. The simulation runs entirely in your browser.
What is a phantom traffic jam?
A jam that appears without an accident, roadworks or a narrowing. When traffic is dense, one driver brakes a little, the driver behind reacts a moment later and brakes a little harder, and so on, until cars stop. The stop then moves backwards against the traffic. The Phantom traffic jam preset shows it on a ring road.
How do the cars decide how to drive?
Each car follows the car ahead with the Intelligent Driver Model, a standard model in traffic research: it wants to reach its desired speed, keeps a safe time gap and brakes smoothly. Drivers see with a reaction time and a little imprecision. Lane changes follow the MOBIL rule, which changes lanes when it gains speed without making others brake hard. At the roundabout drivers wait for a big enough gap.
What do flow, density and delay mean?
Flow is how many vehicles pass a point per hour. Density is how many vehicles there are per kilometre of one lane. Speed, flow and density are linked: flow equals density times average speed. Mean delay is the time a vehicle loses compared with driving at the speed limit, including time spent waiting to enter.
How do I read the time-space diagram?
Time runs to the right and position along the road runs upwards, with every car drawn in the colour of its speed. A car driving freely draws a green line rising to the right. A jam is a red band; when the band slopes down to the right, the jam is moving backwards while the cars inside it move forwards.
Why does zipper merging help?
If drivers on the ramp use the whole merge lane and join one by one at its end, both lanes are used up to the merge point and nobody has to stop to squeeze into the first gap. Compare the Zipper merge and Early merge presets: same traffic, different mean delay.
Can I repeat or share an experiment?
Yes. Runs are seeded: the same seed and settings give exactly the same traffic, so you can change one thing at a time. Save a PNG of the road or export an MP4 or WebM video in wide, square or vertical shape.
Can I use it for teaching?
Yes. The presets are short lessons on phantom jams, reaction time, merging, signal timing and roundabouts, and the live numbers and the diagram make the effects easy to discuss in class.
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